Semiconductor Device, Display Device, And Electronic Device

ABSTRACT

A display device includes a load, a transistor for controlling a current value supplied to the load, a capacitor, a first wiring, a second wiring, and first to fourth switches. Variations in the current value caused by variations in the threshold voltage of the transistor can be suppressed through the steps of: (1) holding the threshold voltage of the transistor in the storage capacitor, (2) inputting a potential in accordance with a video signal, and (3) holding a voltage that is the sum of the threshold voltage and the potential in accordance with the video signal, in the storage capacitor. Accordingly, a desired current can be supplied to the load such as a light emitting element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/604,750, filed Sep. 6, 2012, now allowed, which is a divisional ofU.S. application Ser. No. 12/713,391, filed Feb. 26, 2010, now U.S. Pat.No. 8,264,430, which is a continuation of U.S. application Ser. No.11/563,880, filed Nov. 28, 2006, now U.S. Pat. No. 7,671,826, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 2005-349780 on Dec. 2, 2005, all of which are incorporated byreference.

TECHNICAL FIELD

The present invention relates to a semiconductor device having afunction of controlling a current supplied to a load with a transistor,and a display device which includes a pixel formed using a current-drivedisplay element whose luminance changes in accordance with signals, anda signal line driver circuit and a scan line driver circuit which drivethe pixel. The invention also relates to a driving method of such asemiconductor device and display device. Further, the invention relatesto an electronic device having the display device in a display portion.

BACKGROUND ART

In recent years, a self-luminous display device having a pixel formedusing a light emitting element such as an electroluminescent (EL)element, i.e., a light emitting device has attracted attention. As alight emitting element used for such a self-luminous display device, anorganic light emitting diode (OLED) and an EL element have attractedattention, which have been used for an EL display or the like. Sincethese light emitting elements emit light by themselves, they haveadvantages over a liquid crystal display in higher pixel visibility, nobacklight required, and higher response speed. Note that the luminanceof most of light emitting elements is controlled by a current valueflowing to the light emitting element.

In addition, development of an active matrix display device has beenadvanced, in which each pixel is provided with a transistor forcontrolling light emission of a light emitting element. The activematrix display device is expected to be put into practical use becausenot only can it achieve high-definition and large-screen display that isdifficult for a passive matrix display device, but also it operates withless power consumption than a passive matrix display device.

A pixel configuration of a conventional active matrix display device isshown in FIG. 45 (Reference 1: Japanese Published Patent Application No.H8-234683). The pixel shown in FIG. 45 includes thin film transistors(TFTs) 11 and 12, a capacitor 13, and a light emitting element 14, andis connected to a signal line 15 and a scan line 16. Note that either asource electrode or a drain electrode of the TFT 12 and one electrode ofthe capacitor 13 are supplied with a power supply potential Vdd, and anopposite electrode of the light emitting element 14 is supplied with aground potential.

When amorphous silicon is used for a semiconductor layer of the TFT 12which controls a current value supplied to the light emitting element,that is, a drive TFT, fluctuations of the threshold voltage (Vth) occurdue to deterioration or the like. In that case, although the samepotential is applied to different pixels through the signal line 15, acurrent flowing to the light emitting element 14 differs from pixel topixel and the resulting luminance becomes nonuniform among pixels. Notethat in the case of using polysilicon for the semiconductor layer of thedrive TFT, characteristics of the transistor deteriorate or varylikewise.

In order to overcome the above problem, an operating method using apixel in FIG. 46 is proposed in Reference 2 (Reference 2: JapanesePublished Patent Application No. 2004-295131). The pixel shown in FIG.46 includes a transistor 21, a drive transistor 22 which controls acurrent value supplied to a light emitting element 24, a capacitor 23,and the light emitting element 24, and the pixel is connected to asignal line 25 and a scan line 26. Note that the drive transistor 22 isan NMOS transistor. Either a source electrode or a drain electrode ofthe drive transistor 22 is supplied with a ground potential, and anopposite electrode of the light emitting element 24 is supplied withVca.

FIG. 47 shows a timing chart of the operation of this pixel. In FIG. 47,one frame period is divided into an initialization period 31, athreshold (Vth) write period 32, a data write period 33, and a lightemitting period 34. Note that one frame period corresponds to a periodfor displaying an image for one screen, and the initialization period,the threshold (Vth) write period, and the data write period arecollectively referred to as an address period.

First, in the threshold write period 32, the threshold voltage of thedrive transistor 22 is written into the capacitor 23. After that, in thedata write period 33, a data voltage (Vdata) indicative of the luminanceof the pixel is written into the capacitor 23, and thus Vdata+Vth isaccumulated in the capacitor 23. Then, in the light emitting period 34,the drive transistor 22 is turned on, so that the light emitting element24 emits light at a luminance specified by the data voltage by changingVca. Such operation can reduce luminance variations caused byfluctuations of the threshold voltage of the drive transistor.

Reference 3 also discloses that a gate-source voltage of a drive TFT isset at a voltage corresponding to the sum of a data potential and thethreshold voltage of the drive TFT, and thus a current flowing to alight-emitting element does not change even when the threshold voltageof the TFT fluctuates (Reference 3: Japanese Published PatentApplication No. 2004-280059).

In each of the operating methods disclosed in References 2 and 3, theinitialization, the threshold voltage writing, and the light emissionare performed by changing a potential of Vca several times in each frameperiod. In these pixels, one electrode of a light emitting element whichis supplied with a potential Vca, that is, an opposite electrode isformed entirely over the pixel region. Therefore, the light emittingelement cannot emit light if there is even a single pixel in which datawriting operation is performed besides initialization and thresholdvoltage writing. Thus, the ratio of a light emitting period to one frameperiod (i.e., duty ratio) becomes low as shown in FIG. 48.

When the duty ratio is low, the amount of current supplied to alight-emitting element through a driving transistor has to be increased;therefore, a voltage applied to the light-emitting element becomeshigher, which results in high power consumption. Further, since thelight-emitting element and the driving transistor will easily degradewith a low duty ratio, even higher power is required for obtaining aboutthe same level of luminance as that before degradation.

In addition, since the opposite electrode is connected to all of thepixels, the light-emitting element functions as an element with largecapacitance. Accordingly, in order to change the potential of theopposite electrode, high power consumption is required.

DISCLOSURE OF INVENTION

In view of the foregoing problems, it is an object of the invention toprovide a display device with low power consumption and a high dutyratio. It is another object of the invention to provide a pixelconfiguration, a semiconductor device, and a display device in which aluminance deviation from the level specified by a data potential issmall.

Note that the invention is directed not only to a display device havinga light-emitting element and, therefore, it is still another object ofthe invention to suppress variations in the current value which resultfrom variations in the threshold voltage of a transistor. Accordingly, adestination of a current which is controlled with a driving transistoris not limited to the light-emitting element.

One aspect of the invention provides a semiconductor device having apixel which includes a transistor, a first switch, a second switch, afirst wiring, and a second wiring. One of a source electrode and a drainelectrode of the transistor is electrically connected to a pixelelectrode and the second switch; the other of the source electrode andthe drain electrode of the transistor is electrically connected to thefirst wiring; and a gate electrode of the transistor is electricallyconnected to the second wiring through the first switch. A signal inaccordance with a gray scale level of the pixel is input to the gateelectrode of the transistor.

One aspect of the invention provides a semiconductor device including atransistor, a storage capacitor, a first switch, a second switch, and athird switch. One of a source electrode and a drain electrode of thetransistor is electrically connected to a pixel electrode and alsoelectrically connected to a third wiring through the third switch; theother of the source electrode and the drain electrode of the transistoris electrically connected to a first wiring; and a gate electrode of thetransistor is electrically connected to a second wiring through thesecond switch and also electrically connected to a fourth wiring throughthe first switch. One of the source electrode and the drain electrode ofthe transistor is electrically connected to the gate electrode of thetransistor through the storage capacitor.

The third wiring may be a wiring selected from three wirings whichcontrol the first to third switches respectively of a preceding row or anext row.

One aspect of the invention provides a semiconductor device including atransistor, a storage capacitor, a first switch, a second switch, athird switch, and a fourth switch. One of a source electrode and a drainelectrode of the transistor is electrically connected to a pixelelectrode and also electrically connected to a third wiring through thethird switch; the other of the source electrode and the drain electrodeof the transistor is electrically connected to a first wiring; and agate electrode of the transistor is electrically connected to a secondwiring through the fourth switch and the second switch and alsoelectrically connected to a fourth wiring through the fourth switch andthe first switch. One of the source electrode and the drain electrode ofthe transistor is electrically connected to the gate electrode of thetransistor through the storage capacitor and the fourth switch.

One aspect of the invention provides a semiconductor device including atransistor, a storage capacitor, a first switch, a second switch, athird switch, and a fourth switch. One of a source electrode and a drainelectrode of the transistor is electrically connected to a pixelelectrode and also electrically connected to a third wiring through thethird switch; the other of the source electrode and the drain electrodeof the transistor is electrically connected to a first wiring; and agate electrode of the transistor is electrically connected to a secondwiring through the second switch and also electrically connected to afourth wiring through the fourth switch and the first switch. One of thesource electrode and the drain electrode of the transistor iselectrically connected to the gate electrode of the transistor throughthe storage capacitor and the fourth switch.

One aspect of the invention provides a semiconductor device including atransistor, a storage capacitor, a first switch, a second switch, athird switch, and a fourth switch. One of a source electrode and a drainelectrode of the transistor is electrically connected to a pixelelectrode and also electrically connected to a third wiring through thethird switch; the other of the source electrode and the drain electrodeof the transistor is electrically connected to a first wiring throughthe fourth switch; and a gate electrode of the transistor iselectrically connected to a second wiring through the second switch andalso electrically connected to a fourth wiring through the first switch.One of the source electrode and the drain electrode of the transistor iselectrically connected to the gate electrode of the transistor throughthe storage capacitor.

One aspect of the invention provides a semiconductor device including atransistor, a storage capacitor, a first switch, a second switch, athird switch, and a fourth switch. One of a source electrode and a drainelectrode of the transistor is electrically connected to a pixelelectrode through the fourth switch and also electrically connected to athird wiring through the fourth switch and the third switch; the otherof the source electrode and the drain electrode of the transistor iselectrically connected to a first wiring; and a gate electrode of thetransistor is electrically connected to a second wiring through thesecond switch and also electrically connected to a fourth wiring throughthe first switch. One of the source electrode and the drain electrode ofthe transistor is electrically connected to the gate electrode of thetransistor through the fourth switch and the storage capacitor.

The third wiring may be the same as a wiring which controls the thirdswitch.

The third wiring may be a wiring selected from four wirings whichcontrol the first to fourth switches respectively of a preceding row ora next row.

The transistor may be an n-channel transistor. In addition, asemiconductor layer of the transistor may be formed of a non-crystallinesemiconductor film. Further, the semiconductor layer of the transistormay be formed of amorphous silicon.

Alternatively, the semiconductor layer of the transistor may be formedof a crystalline semiconductor film.

In the aforementioned invention, a potential supplied to the secondwiring may be higher than a potential supplied to the third wiring, anda difference between the two potentials may be larger than the thresholdvoltage of the transistor.

The transistor may also be a p-channel transistor. In that case, apotential supplied to the second wiring may be lower than a potentialsupplied to the third wiring, and a difference between the twopotentials may be larger than the absolute value of the thresholdvoltage of the transistor.

One aspect of the invention provides a semiconductor device including atransistor, one of a source electrode and a drain electrode of which iselectrically connected to a first wiring, the other of the sourceelectrode and the drain electrode of which is electrically connected toa third wiring, and a gate electrode of which is electrically connectedto a second wiring and a fourth wiring; a storage capacitor which holdsa gate-source voltage of the transistor; a means for holding a firstvoltage in the storage capacitor by applying to the storage capacitor afirst potential which is supplied to the second wiring and a secondpotential which is supplied to the third wiring; a means for discharginga voltage of the storage capacitor down to a second voltage; a means forholding a fifth voltage that is the sum of the second voltage and afourth voltage in the storage capacitor by applying to the storagecapacitor a potential that is the sum of the first potential and a thirdvoltage; and a means for supplying a load with a current which is setfor the transistor in accordance with the fifth voltage.

One aspect of the invention provides a semiconductor device including atransistor, one of a source electrode and a drain electrode of which iselectrically connected to a first wiring, the other of the sourceelectrode and the drain electrode of which is electrically connected toa third wiring, and a gate electrode of which is electrically connectedto a second wiring and a fourth wiring; a storage capacitor which holdsa gate-source voltage of the transistor; a means for holding a firstvoltage in the storage capacitor by applying to the storage capacitor afirst potential which is supplied to the second wiring and a secondpotential which is supplied to the third wiring; a means for discharginga voltage of the storage capacitor down to the threshold voltage of thetransistor; a means for holding a fourth voltage that is the sum of thethreshold voltage of the transistor and a third voltage in the storagecapacitor by applying to the storage capacitor a potential that is thesum of the first potential and a second voltage; and a means forsupplying a load with a current which is set for the transistor inaccordance with the fourth voltage.

The transistor may be an n-channel transistor. In addition, asemiconductor layer of the transistor may be formed of a non-crystallinesemiconductor film. Further, the semiconductor layer of the transistormay be formed of amorphous silicon.

Alternatively, the semiconductor layer of the transistor may be formedof a crystalline semiconductor film.

In the aforementioned invention, the first potential may be higher thanthe second potential and a difference between the first potential andthe second potential may be larger than the threshold voltage of thetransistor.

The transistor may also be a p-channel transistor. In that case, thefirst potential may be lower than the second potential and a differencebetween the first potential and the second potential may be larger thanthe absolute value of the threshold voltage of the transistor.

One aspect of the invention provides a display device which includes theaforementioned semiconductor device, and also provides an electronicdevice which includes the display device as a display portion.

Note that the switch described in this specification is not particularlylimited and may be either an electrical switch or a mechanical switch aslong as it can control a current flow. The switch may be a transistor, adiode, or a logic circuit combining them. In the case of using atransistor as a switch, the transistor operates as a mere switch.Therefore, the polarity (conductivity type) of the transistor is notparticularly limited. However, it is desirable to use a transistorhaving a characteristic of smaller off-current. As the transistor withsmall off-current, there are a transistor provided with an LDD region, atransistor having a multi-gate structure, and the like. In addition, itis desirable to use an n-channel transistor when a transistor to beoperated as a switch operates in the state that a potential of a sourceelectrode thereof is closer to a low-potential-side power source (e.g.,Vss, GND, 0 V, or the like), whereas it is desirable to use a p-channeltransistor when the transistor operates in the state that a potential ofa source electrode thereof is closer to a high-potential-side powersource (e.g., Vdd or the like). This is because the absolute value of agate-source voltage can be increased, so that the transistor easilyoperates as a switch. Note that the switch may be a CMOS circuit whichuses both an n-channel transistor and a p-channel transistor.

Note that the description “being connected” in the invention issynonymous with “being electrically connected”. Thus, another element,switch, or the like may be interposed.

Note also that the load may be any element. For example, a displaymedium whose contrast varies by an electromagnetic action can be used,such as a light emitting element including an EL element (e.g., anorganic EL element, an inorganic EL element, or an EL element containingan organic material and an inorganic material) and an electron emittingelement as well as a liquid crystal element or electronic ink. Note thata display device using an electron emitting element includes a fieldemission display (FED), an SED flat-panel display (SED:Surface-conduction Electron-emitter Display), and the like. In addition,a display device using electronic ink includes electronic paper.

The transistor applicable to the invention is not particularly limited,and it may be a thin film transistor (TFT) using a non-singlecrystalline semiconductor film typified by an amorphous silicon film ora polycrystalline silicon film, a transistor formed using asemiconductor substrate or an SOI substrate, a MOS transistor, ajunction transistor, a bipolar transistor, a transistor using an organicsemiconductor or a carbon nanotube, or other transistors. In addition,the substrate over which the transistor is formed is not particularlylimited, and the transistor can be formed over a single crystallinesubstrate, an SOI substrate, a glass substrate, a plastic substrate, orthe like.

Note that as described above, the transistor in the invention may be ofany type and may be formed over any type of substrate. Accordingly, allcircuits may be formed over a glass substrate, a plastic substrate, asingle crystalline substrate, an SOI substrate, or any other substrates.Alternatively, a part of the circuits may be formed over a substrate,and another part of the circuits may be formed over another substrate.That is, not all of the circuits are not required to be formed over thesame substrate. For example, a part of the circuits may be formed over aglass substrate using TFTs, and another part of the circuits may beformed on an IC chip using a single crystalline substrate, so that theIC chip is connected onto the glass substrate by COG (Chip On Glass).Alternatively, the IC chip may be connected to the glass substrate byTAB (Tape Automated Bonding) or using a printed circuit board.

In this specification, one pixel means one color element. Accordingly,in the case of a full-color display device including R (red), G (green),and B (blue) color elements, one pixel means any one of R, G, and Bcolor elements.

Note that the description “pixels are arranged in matrix” in thisspecification includes not only the case where pixels are arranged in agrid pattern which is a combination of vertical stripes and horizontalstripes, but also the case where, when full-color display is performedwith three color elements (e.g. RGB), pixels of three color elementswhich constitute the smallest unit of an image are arranged in aso-called delta pattern. In addition, the size of each pixel may bedifferent from one another according to color elements.

Note that a “semiconductor device” in this specification means a devicehaving a circuit which includes a semiconductor element (such as atransistor or a diode). In addition, a “display device” includes notonly the main body of a display panel in which a plurality of pixelseach including a load and a peripheral driver circuit for driving thepixels are formed over a substrate but also a display panel with aflexible printed circuit (FPC) or a printed wiring board (PWB) attachedthereto.

According to the invention, variations in the current value caused byvariations in the threshold voltage of transistors can be suppressed.Therefore, a desired current can be supplied to a load such as a lightemitting element. In particular, when a light emitting element is usedas a load, a display device with few luminance variations and a highduty ratio can be provided.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings,

FIG. 1 shows a pixel configuration shown in Embodiment Mode 1;

FIG. 2 is a timing chart illustrating the operation of the pixel shownin FIG. 1;

FIGS. 3A to 3D illustrate the operation of the pixel shown in FIG. 1;

FIG. 4 is a model diagram of the voltage-current characteristics inaccordance with the channel length modulation;

FIG. 5 illustrates a pixel configuration shown in Embodiment Mode 1;

FIG. 6 illustrates a pixel configuration shown in Embodiment Mode 1;

FIG. 7 illustrates a pixel configuration shown in Embodiment Mode 1;

FIG. 8 illustrates a pixel configuration shown in Embodiment Mode 1;

FIG. 9 illustrates a display device shown in Embodiment Mode 1;

FIG. 10 is a chart illustrating the write operation of a display deviceshown in Embodiment Mode 1;

FIG. 11 illustrates a pixel configuration shown in Embodiment Mode 2;

FIG. 12 illustrates a pixel configuration shown in Embodiment Mode 4;

FIG. 13 illustrates a pixel configuration shown in Embodiment Mode 4;

FIG. 14 illustrates a pixel configuration shown in Embodiment Mode 4;

FIG. 15 illustrates a pixel configuration shown in Embodiment Mode 4;

FIGS. 16A to 16F illustrate pixel configurations shown in EmbodimentMode 3;

FIG. 17 is a fragmentary sectional view of a pixel shown in EmbodimentMode 8;

FIGS. 18A and 18B illustrate light-emitting elements shown in EmbodimentMode 8;

FIGS. 19A to 19C illustrate directions of light emission shown inEmbodiment Mode 8;

FIGS. 20A and 20B are fragmentary sectional views of a pixel shown inEmbodiment Mode 8;

FIGS. 21A and 21B are fragmentary sectional views of a pixel shown inEmbodiment Mode 8;

FIGS. 22A and 22B are fragmentary sectional views of a pixel shown inEmbodiment Mode 8;

FIG. 23 is a fragmentary sectional view of a pixel shown in EmbodimentMode 8;

FIG. 24 is a fragmentary sectional view of a pixel shown in EmbodimentMode 8;

FIGS. 25A and 25B each illustrate a display device shown in EmbodimentMode 9;

FIGS. 26A and 26B each illustrate a display device shown in EmbodimentMode 9;

FIGS. 27A and 27B each illustrate a display device shown in EmbodimentMode 9;

FIG. 28 is a fragmentary sectional view of a pixel shown in EmbodimentMode 9;

FIG. 29 illustrates a pixel configuration shown in Embodiment Mode 5;

FIG. 30 illustrates a pixel configuration shown in Embodiment Mode 5;

FIG. 31 illustrates a pixel configuration shown in Embodiment Mode 6;

FIG. 32 is a timing chart illustrating the operation of the pixel shownin FIG. 31;

FIGS. 33A to 33H are views of electronic devices to which the inventioncan be applied;

FIG. 34 shows an exemplary configuration of a mobile phone;

FIG. 35 shows an example of an EL module;

FIG. 36 is a block diagram showing the main configuration of an ELtelevision receiver;

FIG. 37 illustrates a pixel configuration shown in Embodiment Mode 6;

FIG. 38 illustrates a pixel configuration shown in Embodiment Mode 6;

FIG. 39 illustrates a pixel configuration shown in Embodiment Mode 7;

FIG. 40 is a timing chart illustrating the operation of the pixel shownin FIG. 39;

FIGS. 41A to 41D illustrate the operation of the pixel shown in FIG. 39;

FIG. 42 illustrates a pixel configuration shown in Embodiment Mode 2;

FIG. 43 is a top view of the pixel shown in FIG. 11;

FIG. 44 is a top view of the pixel shown in FIG. 11;

FIG. 45 illustrates a pixel configuration using a conventionaltechnique;

FIG. 46 illustrates a pixel configuration using a conventionaltechnique;

FIG. 47 is a timing chart for operating the pixel using a conventionaltechnique;

FIG. 48 illustrates the ratio of a light-emitting period to one frameperiod in the case of using the conventional technique; and

FIG. 49 illustrates a driving scheme which combines a digital gray scalemethod and a time gray scale method.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiment modes and embodiments of the invention will bedescribed. Note that it is easily understood by a person skilled in theart that the invention can be embodied in many different modes and canbe changed in various ways without departing from the spirit and thescope of the invention. Therefore, the invention should not be construedas being limited the following description. Note that like referencenumerals are used to denote like elements throughout the drawings whichillustrate the structures of the invention.

Embodiment Mode 1

A basic configuration of a pixel of the invention is described withreference to FIG. 1. The pixel shown in FIG. 1 includes a transistor110, a first switch 111, a second switch 112, a third switch 113, afourth switch 114, a capacitor 115, and a light emitting element 116.The pixel is connected to a signal line 117, a first scan line 118, asecond scan line 119, a third scan line 120, a fourth scan line 121, afirst potential supply line 122, a second potential supply line 123, anda power supply line 124. In this embodiment mode, the transistor 110 isan n-channel transistor which is turned on when a gate-source voltage(Vgs) thereof exceeds the threshold voltage (Vth). In addition, a pixelelectrode of the light emitting element 116 is an anode and an oppositeelectrode 125 thereof is a cathode. Note that the gate-source voltage ofthe transistor is represented by Vgs; the drain-source voltage, Vds; thethreshold voltage, Vth; and a voltage accumulated in the capacitor, Vcs.In addition, the power supply line 124, the first potential supply line122, the second potential supply line 123, and the signal line 117 arealso referred to as a first wiring, a second wiring, a third wiring, anda fourth wiring, respectively.

A first electrode (one of a source electrode and a drain electrode) ofthe transistor 110 is connected to the pixel electrode of the lightemitting element 116; a second electrode (the other of the sourceelectrode and the drain electrode) thereof, to the power supply line124; and a gate electrode thereof, to the first potential supply line122 through the fourth switch 114 and the second switch 112. Note thatthe fourth switch 114 is connected between the gate electrode of thetransistor 110 and the second switch 112. When a connection point of thefourth switch 114 and the second switch 112 is denoted by a node 130,the node 130 is connected to the signal line 117 through the firstswitch 111. In addition, the first electrode of the transistor 110 isalso connected to the second potential supply line 123 through the thirdswitch 113.

Further, the capacitor 115 is connected between the node 130 and thefirst electrode of the transistor 110. That is, a first electrode of thecapacitor 115 is connected to the gate electrode of the transistor 110through the fourth switch 114, while a second electrode of the capacitor115 is connected to the first electrode of the transistor 110. Thecapacitor 115 may be formed by sandwiching an insulating film between awiring, a semiconductor layer, and an electrode or can be omitted byutilizing the gate capacitance of the transistor 110. Such a means forholding a voltage is called a storage capacitor. Note that a connectionpoint of the node 130, the first switch 111, and the first electrode ofthe capacitor 115 is denoted by a node 131, and a connection point ofthe first electrode of the transistor 110, the second electrode of thecapacitor 115, and the pixel electrode of the light emitting element 116is denoted by a node 132.

Note that on/off of the first switch 111, the second switch 112, thethird switch 113, and the fourth switch 114 is controlled by inputtingsignals to the first scan line 118, the second scan line 119, the thirdscan line 120, and the fourth scan line 121, respectively.

A signal in accordance with a gray scale level of the pixel whichcorresponds to a video signal, i.e., a potential in accordance withluminance data is input to the signal line 117.

Next, operation of the pixel shown in FIG. 1 is described with referenceto a timing chart in FIG. 2 and FIGS. 3A to 3D. Note that one frameperiod which corresponds to a period for displaying an image for onescreen is divided into an initialization period, a threshold writeperiod, a data write period, and a light emitting period in FIG. 2. Theinitialization period, the threshold write period, and the data writeperiod are collectively referred to as an address period. The length ofone frame period is not particularly limited, but is preferably 1/60second or less so that an image viewer does not perceive flickers.

A potential V1 is input to the opposite electrode 125 of the lightemitting element 116 and the first potential supply line 122, while apotential V1−Vth−α (α: an arbitrary positive number) is input to thesecond potential supply line 123. In addition, a potential V2 is inputto the power supply line 124.

Here, the potential of the opposite electrode 125 of the light emittingelement 116 is set equal to the potential of the first potential supplyline 122 for descriptive purposes. However, given that the minimumpotential difference which is necessary for the light emitting element116 to emit light is represented by V_(EL), it is acceptable as long asthe potential of the opposite electrode 125 is higher than a potentialV1−Vth−α−V_(EL). In addition, it is acceptable as long as the potentialV2 of the power supply line 124 is higher than the sum of the potentialof the opposite electrode 125 and the minimum potential difference(V_(EL)) which is necessary for the light emitting element 116 to emitlight. However, since the potential of the opposite electrode 125 is setat V1 here for descriptive purposes, it is acceptable as long as V2 ishigher than V1+V_(EL).

First, the first switch 111 is turned off while the second switch 112,the third switch 113, and the fourth switch 114 are turned on in theinitialization period as shown in (A) in FIG. 2 and FIG. 3A. At thistime, the first electrode of the transistor 110 serves as a sourceelectrode, and a potential thereof is equal to the potential of thesecond potential supply line 123 which is V1−Vth−α. On the other hand, apotential of the gate electrode of the transistor 110 is V1. Thus, thegate-source voltage Vgs of the transistor 110 is Vth+α and thus thetransistor 110 is turned on. Then, Vth+α is held in the capacitor 115which is provided between the gate electrode and the first electrode ofthe transistor 110. Although the fourth switch 114 shown herein is in anon state, it may be an off state.

Next, the third switch 113 is turned off in the threshold write periodshown in (B) in FIG. 2 and FIG. 3B. Therefore, the potential of thefirst electrode, i.e., the source electrode of the transistor 110 risesgradually and when it reaches V1−Vth, in other words, when thegate-source voltage Vgs of the transistor 110 reaches the thresholdvoltage (Vth), the transistor 110 is turned off. Thus, the voltage heldin the capacitor 115 becomes Vth.

In the next data write period shown in (C) in FIG. 2 and FIG. 3C, thesecond switch 112 and the fourth switch 114 are turned off, and then thefirst switch 111 is turned on so that a potential (V1+Vdata) inaccordance with luminance data is input from the signal line 117. Notethat the transistor 110 can be kept in an off state by turning off thefourth switch 114. Therefore, potential fluctuations of the secondelectrode of the capacitor 115, which result from a current suppliedfrom the power supply line 124 at data writing, can be suppressed. Atthis time, the voltage Vcs held in the capacitor 115 can be representedby Formula (1) where capacitances of the capacitor 115 and the lightemitting element 116 are C1 and C2, respectively.

$\begin{matrix}{{Vcs} = {{Vth} + {{Vdata} \times \frac{C\; 2}{{C\; 1} + {C\; 2}}}}} & (1)\end{matrix}$

Note that C2>>C1 because the light emitting element 116 is thinner andhas a larger electrode area than the capacitor 115. Thus, fromC2/(C1+C2)≈1, the voltage Vcs held in the capacitor 115 is representedby Formula (2). Note that when the light emitting element 116 iscontrolled not to emit light in the next light-emitting period, apotential of Vdata≦0 is input.

Vcs=Vth+Vdata  (2)

Next, in the light emitting period shown in (D) in FIG. 2 and FIG. 3D,the first switch 111 is turned off and the fourth switch 114 is turnedon. At this time, the gate-source voltage Vgs of the transistor 110 isequal to Vth+Vdata, and thus the transistor 110 is turned on dependingon the value of Vdata. Then, a current in accordance with luminance dataflows to the transistor 110 and the light emitting element 116, so thatthe light emitting element 116 emits light.

Note that a current I flowing to the light emitting element isrepresented by Formula (3) when the transistor 110 is operated in thesaturation region.

$\begin{matrix}\begin{matrix}{I = {\frac{1}{2}\left( \frac{W}{L} \right)\mu \; {{Cox}\left( {{Vgs} - {Vth}} \right)}^{2}}} \\{= {\frac{1}{2}\left( \frac{W}{L} \right)\mu \; {{Cox}\left( {{Vth} + {Vdata} - {Vth}} \right)}^{2}}} \\{= {\frac{1}{2}\left( \frac{W}{L} \right)\mu \; {{Cox}({Vdata})}^{2}}}\end{matrix} & (3)\end{matrix}$

In addition, a current I flowing to the light emitting element isrepresented by Formula (4) when the transistor 110 is operated in thelinear region.

$\begin{matrix}\begin{matrix}{I = {\left( \frac{W}{L} \right)\mu \; {{Cox}\left\lbrack {{\left( {{Vgs} - {Vth}} \right){Vds}} - {\frac{1}{2}{Vds}^{2}}} \right\rbrack}}} \\{= {\left( \frac{W}{L} \right)\mu \; {{Cox}\left\lbrack {{\left( {{Vth} + {Vdata} - {Vth}} \right){Vds}} - {\frac{1}{2}{Vds}^{2}}} \right\rbrack}}} \\{= {\left( \frac{W}{L} \right)\mu \; {{Cox}\left\lbrack {{({Vdata}){Vds}} - {\frac{1}{2}{Vds}^{2}}} \right\rbrack}}}\end{matrix} & (4)\end{matrix}$

In the formulas, W is the channel width of the transistor 110; L, thechannel length; μ, mobility; and Cox, accumulated capacitance.

According to Formulas (3) and (4), the current flowing to the lightemitting element 116 does not depend on the threshold voltage (Vth) ofthe transistor 110 regardless of the operation region of the transistor110, i.e., the saturation region or the linear region. Therefore,variations in the current value caused by variations in the thresholdvoltage of the transistor 110 can be suppressed and a current value inaccordance with luminance data can be supplied to the light emittingelement 116.

Accordingly, variations in luminance caused by variations in thethreshold voltage of the transistor 110 can be suppressed. In addition,power consumption can be reduced because the operation is performed withthe opposite electrode fixed at a constant potential.

Furthermore, when the transistor 110 is operated in the saturationregion, it is also possible to suppress variations in luminance causedby deterioration of the light emitting element 116. When the lightemitting element 116 deteriorates, V_(EL) of the light emitting element116 increases and the potential of the first electrode, i.e., the sourceelectrode of the transistor 110 rises accordingly. At this time, thesource electrode of the transistor 110 is connected to the secondelectrode of the capacitor 115; the gate electrode of the transistor 110is connected to the first electrode of the capacitor 115; and the gateelectrode side is in a floating state. Therefore, in accordance with anincrease in the source potential, the gate potential of the transistor110 also increases by the same amount. Thus, Vgs of the transistor 110does not change. Therefore, the current flowing to the transistor 110and the light emitting element 116 is not affected even if the lightemitting element deteriorates. Note that it can also be seen in Formula(3) that the current I flowing to the light emitting element does notdepend on the source potential or the drain potential.

Therefore, when the transistor 110 is operated in the saturation region,it is possible to suppress variations in the current value flowing tothe transistor 110 caused by variations in the threshold voltage of thetransistor 110 and deterioration of the light emitting element 116.

Note that when the transistor 110 is operated in the saturation region,as the channel length L is shorter, a larger amount of current willeasily flow through the transistor 110 by significantly increasing thedrain voltage by avalanche breakdown.

When the drain voltage is increased to exceed a pinch-off voltage, apinch-off point moves to the source side and an effective channel lengthwhich substantially functions as a channel decreases. This increases acurrent value, and such a phenomenon is called a channel lengthmodulation. Note that the pinch-off point is a boundary portion at whichthe channel disappears and the thickness of the channel below the gatein that portion is 0. In addition, the pinch-off voltage means a voltagewhen the pinch-off point is at the drain edge. This phenomenon will alsooccur more easily as the channel length L is shorter. For example, amodel diagram of the voltage-current characteristics in accordance withthe channel length modulation is shown in FIG. 4. Note that the channellengths L of transistors (a), (b), and (c) satisfy (a)>(b)>(c) in FIG.4.

Accordingly, in the case of operating the transistor 110 in thesaturation region, the current I with respect to the drain-sourcevoltage Vds is preferably as constant as possible. Thus, the channellength L of the transistor 110 is preferably longer. For example, thechannel length L of the transistor is preferably larger than the channelwidth W thereof. In addition, the channel length L is preferably in therange of 10 to 50 μm inclusive, and more preferably in the range of 15to 40 μm inclusive. However, the channel length L and the channel widthW are not limited to such range.

In addition, since a reverse bias voltage is applied to the lightemitting element 116 in the initialization period, a shorted portion ofthe light emitting element can be insulated and deterioration of thelight emitting element can be suppressed. Thus, the lifetime of thelight emitting element can be extended.

Note that since variations in the current value caused by variations inthe threshold voltage of the transistor can be suppressed, a supplydestination of the current controlled by the transistor is notparticularly limited. Therefore, an EL element (an organic EL element,an inorganic EL element, or an EL element containing an organic materialand an inorganic material), an electron emitting element, a liquidcrystal element, electronic ink, or the like can be used as the lightemitting element 116 shown in FIG. 1.

In addition, it is acceptable as long as the transistor 110 has afunction of controlling a current value supplied to the light emittingelement 116, and the kind of the transistor is not particularly limited.Therefore, a thin film transistor (TFT) using a crystallinesemiconductor film, a thin film transistor using a non-singlecrystalline semiconductor film typified by an amorphous silicon film ora polycrystalline silicon film, a transistor formed using asemiconductor substrate or an SOI substrate, a MOS transistor, ajunction transistor, a bipolar transistor, a transistor using an organicsemiconductor or a carbon nanotube, or other transistors can be used.

The first switch 111 is selected in the timing of inputting a signal inaccordance with a gray scale level of the pixel to the capacitor andcontrols a signal supplied to the gate electrode of the transistor 110.The second switch 112 is selected in the timing of applying apredetermined potential to the gate electrode of the transistor 110 andcontrols whether or not to supply the predetermined potential to thegate electrode of the transistor 110. The third switch 113 is selectedin the timing of applying a predetermined potential for initializing apotential written in the capacitor 115 and decreases the potential ofthe first electrode of the transistor 110. The fourth switch 114controls the connection between the gate electrode of the transistor 110and the capacitor 115. Therefore, the first switch 111, the secondswitch 112, the third switch 113, and the fourth switch 114 are notparticularly limited as long as they have the above functions. Forexample, each of the switches may be a transistor, a diode, or a logiccircuit combining them. Note that the first to third switches are notparticularly necessary if the signal or potential can be applied to thepixel at the above timing. In addition, Embodiment Mode 2 describes thecase where the fourth switch can be omitted.

FIG. 5 shows the case of employing n-channel transistors for the firstswitch 111, the second switch 112, the third switch 113, and the fourthswitch 114. Note that portions common to FIGS. 1 and 5 are denoted bycommon reference numerals, and thus description thereof is omitted.

A first switching transistor 511 corresponds to the first switch 111; asecond switching transistor 512, the second switch 112; a thirdswitching transistor 513, the third switch 113; and a fourth switchingtransistor 514, the fourth switch 114. Note that the channel length ofthe transistor 110 is preferably larger than that of any of the firstswitching transistor 511, the second switching transistor 512, the thirdswitching transistor 513, and the fourth transistor 514.

A gate electrode of the first switching transistor 511 is connected to afirst scan line 118; a first electrode thereof, to a signal line 117;and a second electrode thereof, to a node 131.

In addition, a gate electrode of the second switching transistor 512 isconnected to a second scan line 119; a first electrode thereof, to afirst potential supply line 122; and a second electrode thereof, to anode 130.

A gate electrode of the third switching transistor 513 is connected to athird scan line 120; a first electrode thereof, to a node 132; and asecond electrode thereof, to a second potential supply line 123.

A gate electrode of the fourth switching transistor 514 is connected toa fourth scan line 121; a first electrode thereof, to the gate electrodeof the transistor 110; and a second electrode thereof, to the node 130.

Each switching transistor is turned on when a signal input to each scanline has an H level and turned off when the signal input has an L level.

The pixel configuration in FIG. 5 can also suppress variations in thecurrent value caused by variations in the threshold voltage of thetransistor 110 by using an operating method similar to FIG. 1. Thus, acurrent in accordance with luminance data can be supplied to the lightemitting element 116, and variations in luminance can be suppressed.When the transistor 110 is operated in the saturation region, it is alsopossible to suppress variations in luminance caused by deterioration ofthe light emitting element 116.

Further, a manufacturing process can be simplified because the pixel canbe formed using only n-channel transistors. In addition, anon-crystalline semiconductor such as an amorphous semiconductor or asemi-amorphous semiconductor (also referred to as a microcrystallinesemiconductor) can be used for a semiconductor layer of each transistorincluded in the pixel. For example, amorphous silicon (a-Si:H) can beused as the amorphous semiconductor. By using such a non-crystallinesemiconductor, the manufacturing process can further be simplified.Accordingly, a reduction in manufacturing cost and an improvement inyield can be achieved.

Note that the first switching transistor 511, the second switchingtransistor 512, the third switching transistor 513, and the fourthswitching transistor 514 are operated as mere switches. Therefore, thepolarity (conductivity type) of the transistors is not particularlylimited. However, it is desirable to use a transistor having acharacteristic of smaller off-current. As examples of a transistor withsmall off-current, there are a transistor provided with an LDD region, atransistor having a multi-gate structure, and the like. Alternatively,the switch may be a CMOS circuit which uses both an n-channel transistorand a p-channel transistor.

The fourth switch 114 shown in FIG. 1 may be connected between the node130 and the node 131. Such a configuration is shown in FIG. 6. Thefourth switch 114 in FIG. 1 corresponds to a fourth switch 614, andportions common to FIGS. 1 and 6 are denoted by common referencenumerals, and thus description thereof is omitted.

The pixel configuration in FIG. 6 can also suppress variations in thecurrent value caused by variations in the threshold voltage of thetransistor 110 by using an operating method similar to FIG. 1. Thus, acurrent in accordance with luminance data can be supplied to the lightemitting element 116, and variations in luminance can be suppressed.When the transistor 110 is operated in the saturation region, it is alsopossible to suppress variations in luminance caused by deterioration ofthe light emitting element 116.

The fourth switch 114 shown in FIG. 1 may be provided on the pathbetween the node 132 and the connection point of the second electrode ofthe transistor 110 and the power supply line 124.

One example of such a configuration is illustrated in FIG. 7. In theconfiguration of FIG. 7, the fourth switch 114 in FIG. 1 corresponds toa fourth switch 714, and it is connected between the second electrode ofthe transistor 110 and the power supply line 124. Note that portionscommon to FIGS. 1 and 7 are denoted by common reference numerals, andthus description thereof is omitted.

The current supply to the transistor 110 can be stopped by turning offthe fourth switch 714 even when the transistor 110 is turned on by thefourth switch 714 in the data write period. Therefore, potentialfluctuations of the second electrode of the capacitor 115 in the datawrite period can be suppressed.

Thus, the pixel configuration in FIG. 7 can also suppress variations inthe current value caused by variations in the threshold voltage of thetransistor 110 by using an operating method similar to FIG. 1. Thus, acurrent in accordance with luminance data can be supplied to the lightemitting element 116, and variations in luminance can be suppressed.When the transistor 110 is operated in the saturation region, it is alsopossible to suppress variations in luminance caused by deterioration ofthe light emitting element 116. In addition, when the fourth switch 714is turned off in the initialization period, power consumption can bereduced.

Another example of the pixel configuration is shown in FIG. 8. In FIG.8, the fourth switch 114 in FIG. 1 corresponds to a fourth switch 814,which is connected between the first electrode of the transistor 110 andthe node 132. Note that portions common to FIGS. 1 and 8 are denoted bycommon reference numerals, and thus description thereof is omitted.

The current supply to the node 132 can be stopped by turning off thefourth switch 814 even when the transistor 110 is turned on by thefourth switch 814 in the data write period. Therefore, potentialfluctuations of the second electrode of the capacitor 115 in the datawrite period can be suppressed.

Thus, the pixel configuration in FIG. 8 can also suppress variations inthe current value caused by variations in the threshold voltage of thetransistor 110 by using an operating method similar to FIG. 1. Thus, acurrent in accordance with luminance data can be supplied to the lightemitting element 116, and variations in luminance can be suppressed.When the transistor 110 is operated in the saturation region, it is alsopossible to suppress variations in luminance caused by deterioration ofthe light emitting element 116. In addition, when the fourth switch 814is turned off in the initialization period, power consumption can bereduced.

Note that each of the fourth switch 614, the fourth switch 714, and thefourth switch 814 may be a transistor, a diode, or a logic circuitcombining them, similarly to the first to third switches.

In the case of providing the fourth switch on the path between the node132 and the connection point of the second electrode of the transistor110 and the power supply line 124, as shown in FIGS. 7 and 8, anon-light emission state can be forcibly produced by turning off thefourth switch in the light emitting period. Such operation makes itpossible to freely set the light emitting period. In addition, byinserting black display, afterimages can be made less easily perceivedand moving image characteristics can be increased.

Next, a display device including the pixel of the invention is describedwith reference to FIG. 9.

The display device includes a signal line driver circuit 911, a scanline driver circuit 912, and a pixel portion 913. The pixel portion 913includes a plurality of signal lines S1 to Sm, first potential supplylines P1_1 to Pm_1, and power supply lines P1_3 to Pm_3 which extendfrom the signal line driver circuit 911 in a column direction; aplurality of first scan lines G1_1 to Gn_1, second scan lines G1_2 toGn_2, third scan lines G1_3 to Gn_3, and fourth scan lines G1_4 to Gn_4which extend from the scan line driver circuit 912 in a row direction;and a plurality of pixels 914 which are arranged in matrix correspondingto the signal lines Si to Sm. Further, a plurality of second potentialsupply lines P1_2 to Pn_2 are provided in parallel with the first scanlines G1_1 to Gn_1. Each pixel 914 is connected to a signal line Sj (oneof the signal lines S1 to Sm), a first potential supply line Pj_1, apower supply line Pj_3, a first scan line Gi_1 (one of the scan linesG1_1 to Gn_1), a second scan line Gi_2, a third scan line Gi_3, a fourthscan line Gi_4, and a second potential supply line Pi_2.

Note that the signal line Sj, the first potential supply line Pj_1, thepower supply line Pj_3, the first scan line Gi_1, the second scan lineGi_2, the third scan line Gi_3, the fourth scan line Gi_4, and thesecond potential supply line Pi_2 correspond to the signal line 117, thefirst potential supply line 122, the power supply line 124, the firstscan line 118, the second scan line 119, the third scan line 120, thefourth scan line 121, and the second potential supply line 123,respectively.

In response to signals output from the scan line driver circuit 912, arow of pixels to be operated is selected, and the operation shown inFIG. 2 is performed in each of the pixels in the row. Note that in thedata write period of FIG. 2, a video signal output from the signal linedriver circuit 911 is written into each pixel of the selected row. Atthis time, a potential in accordance with luminance data of each pixelis input to each of the signal lines S1 to Sm.

As shown in FIG. 10, upon terminating a data write period of the i-throw, for example, signal writing to pixels in the (i+1)-th row starts.Note that in order to show the data write period of each row, FIG. 10shows only the operation of the first switch 111 of FIG. 2. In addition,a pixel that has terminated the data write period in the i-th rowproceeds to a light emitting period and emits light in accordance withthe signal written into the pixel.

Thus, start timing of the initialization period can be freely set inrespective rows unless data write periods overlap in the respectiverows. In addition, since each pixel can emit light except in its addressperiod, the ratio of a light emitting period to one frame period (i.e.,duty ratio) can be significantly increased and can be approximately100%. Therefore, a display device with few luminance variations and ahigh duty ratio can be provided.

In addition, since a threshold write period can be set long, thethreshold voltage of the transistor can be written into the capacitormore accurately. Therefore, reliability as a display device is improved.

Note that the configuration of the display device shown in FIG. 9 isonly exemplary, and the invention is not limited to this. For example,the first potential supply lines P1_1 to Pm_1 do not have to be inparallel with the signal lines S1 to Sm, and may be in parallel with thefirst scan lines G1_1 to Gn_1.

Meanwhile, as a driving method of a display device for expressing grayscales, there are an analog gray scale method and a digital gray scalemethod. The analog gray scale method includes a method of controllingthe emission intensity of a light emitting element in an analog mannerand a method of controlling the emission time of a light emittingelement in an analog manner. Between the two, the method of controllingthe emission intensity of a light emitting element in an analog manneris often used. On the other hand, in the digital gray scale method,on/off of a light emitting element is controlled in a digital manner toexpress gray scales. The digital gray scale method has the advantage ofhigh noise resistance because data processing can be performed usingdigital signals. However, since there are only two states of a lightemitting state and a non-light emitting state, only two gray scalelevels can be expressed. Therefore, multiple level gray scale display isattempted by using another method in combination. As a technique formultiple level gray scale display, there are an area gray scale methodin which light emitting areas of pixels are weighted and selected toperform gray scale display, and a time gray scale method in which lightemitting time is weighted and selected to perform gray scale display.

In the case of combining the digital gray scale method and the time grayscale method, one frame period is divided into a plurality of subframeperiods (SFn) as shown in FIG. 49. Each subframe period includes anaddress period (Ta) including an initialization period, a thresholdwrite period, and a data write period and a light emitting period (Ts).Note that subframe periods, the number of which corresponds to thenumber of display bits n, are provided in one frame period. In addition,the ratio of lengths of light emitting periods in respective subframeperiods is set to satisfy 2^((n−1)):2^((n−2)): . . . :2:1, and lightemission or non-light emission of light emitting elements is selected ineach light emitting period, so that gray scales are expressed byutilizing the difference in total light emitting time within one frameperiod. When the total light emitting time in one frame period is long,luminance is high, and when short, luminance is low. Note that FIG. 49shows an example of a 4-bit gray scale, in which one frame period isdivided into four subframe periods and 2⁴=16 gray scale levels can beexpressed by a combination of light emitting periods. Note that it isalso possible to express gray scales by setting the ratio of lengths oflight emitting periods on the basis other than the power-of-two ratio.Further, each subframe period may further be divided.

Note that in the case of attempting multiple level gray scale display byusing the time gray scale method as described above, the length of alight emitting period of a lower-order bit is short. Therefore, whendata write operation is started immediately upon termination of a lightemitting period of a preceding subframe period, it overlaps with thedata write operation of the preceding subframe period. In that case,normal operation cannot be performed. Therefore, by providing the fourthswitch between the node 132 and the connection point of the secondelectrode of the transistor 110 and the power supply line 124 as shownin FIGS. 7 and 8, and turning off the fourth switch in the lightemitting period to forcibly produce a non-light emitting state, itbecomes possible to express light emission which has even a shorterlength than the data write periods required for all rows. Thus, theprovision of the fourth switch is effective not only in the analog grayscale method but also in the method which combines a digital gray scalemethod and a time gray scale method as described above.

Note that variations in the threshold voltage include not only adifference between the threshold voltage of each transistor in pixels,but also include a fluctuation in the threshold voltage of eachtransistor over time. Further, the difference in the threshold voltageof each transistor includes the difference in characteristics that areproduced in the manufacture of the transistor. Note also that thetransistor here means a transistor having a function of supplying acurrent to a load such as a light emitting element.

Embodiment Mode 2

In this embodiment mode, a pixel with a different configuration fromEmbodiment Mode 1 is described with reference to FIG. 11. Note thatportions common to this embodiment mode and the preceding embodimentmode are denoted by common reference numerals, and thus detaileddescription of the same portion or a portion having a similar functionis omitted.

The pixel shown in FIG. 11 includes a transistor 110, a first switch111, a second switch 112, a third switch 113, a capacitor 115, and alight emitting element 116. The pixel is connected to a signal line 117,a first scan line 118, a second scan line 119, a third scan line 120, afirst potential supply line 122, a second potential supply line 123, anda power supply line 124.

A first electrode (one of a source electrode and a drain electrode) ofthe transistor 110 is connected to a pixel electrode of thelight-emitting element 116; a second electrode (the other of the sourceelectrode and the drain electrode) thereof, to the power supply line124; and a gate electrode thereof, to the first potential supply line122 through the second switch 112. In addition, the gate electrode ofthe transistor 110 is also connected to the signal line 117 through thefirst switch 111, and the first electrode thereof is also connected tothe second potential supply line 123 through the third switch 113.

Further, the capacitor 115 is connected between the gate electrode andthe first electrode of the transistor 110. That is, a first electrode ofthe capacitor 115 is connected to the gate electrode of the transistor110, while a second electrode of the capacitor 115 is connected to thefirst electrode of the transistor 110. The capacitor 115 may be formedby sandwiching an insulating film between a wiring, a semiconductorlayer, and an electrode or can be omitted by utilizing the gatecapacitance of the transistor 110.

That is, the pixel shown in FIG. 11 corresponds to the pixel shown inFIG. 1 which has no fourth switch 114. The pixel shown in FIG. 11 isalso operated in accordance with the timing chart in FIG. 2.

Unlike the pixel in FIG. 1, the transistor 110 is turned on upon inputof a potential (V1+Vdata) in accordance with luminance data from thesignal line 117 in the data write period shown in (C) in FIG. 2. Thus, apotential of the second electrode of the capacitor 115 increases.Therefore, a voltage Vcs which is held in the capacitor 115 becomeslower than Vth+Vdata. In such a case, a potential in which the potentialfluctuation of the second electrode of the capacitor 115 (V1+V′ data) istaken into account may be input from the signal line 117.

However, the potential input from the signal line is not necessarilyrequired to be V1+V′ data depending on the difference between thecapacitances of the capacitor 115 and the light emitting element 116.For example, the potential input from the signal line may be V1+Vdatasimilarly to Embodiment Mode 1 if the potential fluctuation of thesecond electrode of the capacitor 115 does not greatly affect thevoltage to be held in the capacitor 115.

As shown in Embodiment Mode 1, the first switch 111 is selected in thetiming of inputting a signal in accordance with a gray scale level ofthe pixel to the capacitor and controls a signal supplied to the gateelectrode of the transistor 110. The second switch 112 is selected inthe timing of applying a predetermined potential to the gate electrodeof the transistor 110 and controls whether or not to supply thepredetermined potential to the gate electrode of the transistor 110. Thethird switch 113 is selected in the timing of applying a predeterminedpotential for initializing a potential written in the capacitor 115 anddecreases the potential of the first electrode of the transistor 110.Therefore, the first switch 111, the second switch 112, and the thirdswitch 113 are not particularly limited as long as they have the abovefunctions. For example, each of the switches may be a transistor, adiode, or a logic circuit combining them. Note that the first to thirdswitches are not particularly necessary if the signal or potential canbe applied to the pixel at the above timing. For example, when a signalin accordance with a gray scale level of the pixel can be input to thegate electrode of the transistor 110, the first switch 111 is notrequired to be provided as shown in FIG. 42. The pixel shown in FIG. 42includes a transistor 110, a second switch 112, a third switch 113, anda pixel electrode 4240. A first electrode (one of a source electrode anda drain electrode) of the transistor 110 is connected to the pixelelectrode 4240 and the third switch 113, and a gate electrode thereof isconnected to the first potential supply line 122 through the secondswitch 112. Note that since a gate capacitance 4215 of the transistor110 is utilized as a storage capacitor, the capacitor 115 in FIG. 11 isnot particularly required. Such a pixel can also suppress variations inthe current value caused by variations in the threshold voltage of thetransistor 110 by operating each switch and supplying a desiredpotential to each electrode in a similar manner to FIG. 11. Thus, adesired current can be supplied to the pixel electrode 4240.

The first potential supply line 122 may be provided in parallel with thefirst scan line 118 and the like. One mode of FIG. 11 with such aconfiguration is shown in the top view of FIG. 43. Note that in FIG. 43,each switch is shown as a switching transistor. The first switch 111,the second switch 112, and the third switch 113 in FIG. 11 correspond toa first switching transistor 4301, a second switching transistor 4302,and a third switching transistor 4303, respectively.

A conductive layer 4310 includes a portion functioning as the first scanline 118 and a portion functioning as a gate electrode of the switchingtransistor 4301. A conductive layer 4311 includes a portion functioningas the signal line 117 and a portion functioning as a first electrode ofthe first switching transistor 4301. A conductive layer 4312 includes aportion functioning as a second electrode of the first switchingtransistor 4301, a portion functioning as the first electrode of thecapacitor 115, and a portion functioning as a first electrode of thesecond switching transistor 4302. A conductive layer 4313 includes aportion functioning as a gate electrode of the second switchingtransistor 4302, and is connected to the second scan line 119 through awiring 4314. A conductive layer 4315 includes a portion functioning asthe first potential supply line 122 and a second electrode of the secondswitching transistor 4302. A conductive layer 4316 includes a portionfunctioning as the gate electrode of the transistor 110, and isconnected to the conductive layer 4312 through a wiring 4317. Aconductive layer 4318 includes a portion functioning as the power supplyline 124 and a portion functioning as the second electrode of thetransistor 110. A conductive layer 4319 includes a portion functioningas the first electrode of the transistor 110, and is connected to apixel electrode 4344 of a light-emitting element. A conductive layer4320 includes a portion functioning as a first electrode of the thirdswitching transistor 4303, and is connected to the pixel electrode 4344.A conductive layer 4321 includes a portion functioning as a secondelectrode of the third switching transistor 4303, and is connected tothe second potential supply line 123. A conductive layer 4322 includes aportion functioning as the third scan line 120 and a portion functioningas a gate electrode of the third switching transistor 4303.

Note that among the respective conductive layers, the portionsfunctioning as the gate electrode, the first electrode, and the secondelectrode of the first switching transistor 4301 are formed to overlapwith a semiconductor layer 4333; the portions functioning as the gateelectrode, the first electrode, and the second electrode of the secondswitching transistor 4302 are formed to overlap with a semiconductorlayer 4334; and the portions functioning as the gate electrode, thefirst electrode, and the second electrode of the third switchingtransistor 4303 are formed to overlap with a semiconductor layer 4335.In addition, the portions functioning as the gate electrode, the firstelectrode, and the second electrode of the transistor 110 are theportion of conductive layers which are formed to overlap with asemiconductor layer 4336. The capacitor 115 is formed in a portion wherethe conductive layer 4312 and the pixel electrode 4334 overlap oneanother.

The conductive layer 4310, the conductive layer 4313, the conductivelayer 4316, the conductive layer 4322, the second scan line 119, and thesecond potential supply line 123 can be formed from the same materialand in the same layer. In addition, the semiconductor layer 4333, thesemiconductor layer 4334, the semiconductor layer 4335, and thesemiconductor layer 4336 can be formed from the same material and in thesame layer. Similarly, the conductive layer 4331, the conductive layer4312, the conductive layer 4315, the conductive layer 4318, theconductive layer 4319, the conductive layer 4320, and the conductivelayer 4321 can be formed from the same material and in the same layer.Further, the wirings 4314, 4317, 4323, and 4324 can be formed from thesame material and in the same layer as the pixel electrode 4344. Notethat the first potential supply line 122 is connected to a firstpotential supply line of an adjacent pixel with the wiring 4324.

Next, FIG. 44 shows a top view of a pixel where the first potentialsupply line 122 is formed in the different layer from that in FIG. 43.Note that portions common to FIGS. 43 and 44 are denoted by commonreference numerals.

A first potential supply line 4222 is formed from the same material andin the same layer as the second scan line 119 and the like. In addition,a portion 4401 functioning as a second electrode of the second switchingtransistor 4302 is formed from the same material and in the same layeras the conductive layer 4312 and the like, and is connected to a firstpotential supply line 4422 through a wiring 4402 which is formed fromthe same material and in the same layer as the pixel electrode 4344.Thus, the top views of the pixel are not limited to those shown in FIGS.43 and 44.

Furthermore, the pixel shown in this embodiment mode can be applied tothe display device of FIG. 9. In that case, start timing of theinitialization period can be freely set in respective rows unless datawrite periods in the respective rows overlap, similarly to EmbodimentMode 1. In addition, since each pixel can emit light except in itsaddress period, the ratio of a light emitting period to one frame period(i.e., duty ratio) can be significantly increased and can beapproximately 100%. Therefore, a display device with few luminancevariations and a high duty ratio can be provided.

In addition, since a threshold write period can be set long, thethreshold voltage of the transistor which controls a current valueflowing to the light emitting element can be written into the capacitormore accurately. Therefore, reliability as a display device is improved.

This embodiment mode can be freely combined with the pixelconfigurations shown in the other embodiment modes besides FIG. 1described above. That is, the fourth switch can also be omitted in thepixels shown in the other embodiment modes.

Embodiment Mode 3

In this embodiment mode, a pixel having a different configuration fromthat in Embodiment Mode 1 is described with reference to FIGS. 16A to16F. Note that portions common to FIGS. 1 and 16A through 16F aredenoted by common reference numerals, and thus detailed description ofthe same portion or a portion having a similar function is omitted.

The pixel shown in FIG. 16A includes a transistor 110, a first switch111, a second switch 112, a fourth switch 114, a capacitor 115, a lightemitting element 116, and a rectifier element 1613. The pixel isconnected to a signal line 117, a first scan line 118, a second scanline 119, a fourth scan line 121, a first potential supply line 122, athird scan line 1620, and a power supply line 124.

In the pixel shown in FIG. 16A, the rectifier element 1613 is used asthe third switch 113 in FIG. 1, and a second electrode of the capacitor115, a first electrode of a transistor 110, and a pixel electrode of thelight emitting element 116 are connected to the third scan line 1620through the rectifier element 1613. That is, the rectifier element 1613is connected so that a current flows from the first electrode of thetransistor 110 to the third scan line 1620. Needless to say, the firstswitch 111, the second switch 112, and the fourth switch 114 may betransistors or the like, as shown in the embodiment mode 1. In addition,the rectifier element 1613 may be a Schottky-barrier diode 1651 shown inFIG. 16B, a PIN diode 1652 shown in FIG. 16C, a PN diode 1653 shown inFIG. 16D, a diode-connected transistor shown in FIG. 16E or 16F, or thelike. Note that in the case of using the transistor 1654 or thetransistor 1655, the polarity thereof has to be selected as appropriateaccording to the direction of a current flow.

A current does not flow through the rectifier element 1613 when anH-level signal is input to the third scan line 1620, while a currentflows through the rectifier element 1613 when an L-level signal isinput. Therefore, when the pixel in FIG. 16A is operated in a similarmanner to the pixel in FIG. 1, an L-level signal is input to the thirdscan line 1620 in the initialization period, and an H-level signal isinput to the third scan line 1620 in the periods other than theinitialization period. Note that since it is necessary for the h-levelsignal not only to flow through the rectifier element 1613 but also tolower the potential of the second electrode of the capacitor 115 down toV1−Vth−α (α is an arbitrary positive number), the potential of theL-level signal is set at V1−Vth−α−β (α is an arbitrary positive number).Note that β refers to the threshold voltage of the rectifier element1613 in the forward bias direction.

In view of the foregoing circumstances, the pixel configuration shown inFIG. 16 can also suppress variations in the current value caused byvariations in the threshold voltage of the transistor 110 by operatingthe pixel in a similar manner to the pixel in FIG. 1. Thus, a current inaccordance with luminance data can be supplied to the light emittingelement 116, thus variations in luminance can be suppressed. Inaddition, when the transistor 110 is operated in the saturation region,it is also possible to suppress variations in luminance caused bydeterioration of the light emitting element 116. Further, the use of therectifier element 1613 can reduce the number of wirings, which resultsin an increase in aperture ratio.

Furthermore, the pixel shown in this embodiment mode can be applied tothe display device of FIG. 9. In that case, start timing of theinitialization period can be freely set in respective rows unless datawrite periods in the respective rows overlap, similarly to EmbodimentMode 1. In addition, since each pixel can emit light except in itsaddress period, the ratio of a light emitting period to one frame period(i.e., duty ratio) can be significantly increased and can beapproximately 100%. Therefore, a display device with few luminancevariations and a high duty ratio can be provided.

In addition, since a threshold write period can be set long, thethreshold voltage of the transistor which controls a current valueflowing to the light emitting element can be written into the capacitormore accurately. Therefore, reliability as a display device is improved.

This embodiment mode can be freely combined with the pixelconfigurations shown in the other embodiment modes besides FIG. 1described above. For example, the fourth switch 114 may be connectedbetween the node 130 and the node 131, or between the first electrode ofthe transistor 110 and the node 132. Further, the second electrode ofthe transistor 110 may be connected to the power supply line 124 throughthe fourth switch 114. That is, the rectifier element 1613 can beapplied to the pixels in the other embodiment modes.

Embodiment Mode 4

In this embodiment mode, a pixel having a different configuration fromthat in Embodiment Mode 1 is described with reference to FIGS. 12 to 15.Note that portions common to Embodiment Mode 1 and this embodiment modeare denoted by common reference numerals, and thus detailed descriptionof the same portion or a portion having a similar function is omitted.

A pixel 1200 shown in FIG. 12 includes a transistor 110, a first switch111, a second switch 112, a third switch 113, a fourth switch 114, acapacitor 115, and a light emitting element 116. The pixel is connectedto a signal line 117, a first scan line 1218, a second scan line 119, athird scan line 120, a fourth scan line 121, a first potential supplyline 122, a power supply line 124, and a first scan line 1218 of a nextrow.

Although the first electrode of the transistor 110 shown in FIG. 1 isconnected to the second potential supply line 123 through the thirdswitch 113 in Embodiment Mode 1, the first electrode of the transistor110 can be connected to the first scan line 1218 of a next row as shownin FIG. 12. This is because, it is only necessary that a predeterminedpotential be supplied to the first electrode of the transistor 110 inthe initialization period even without using the second potential supplyline 123. Therefore, as long as a predetermined potential can besupplied to the first electrode of the transistor 110 from a certainwiring in the initialization period, the wiring for supplying thepotential is not required to have a constant potential level at alltimes. Thus, the first scan line 1218 of the next row can be usedinstead of the second potential supply line. In this manner, when awiring of the next row is also used as a wiring for a different purpose,the number of wirings can be reduced and thus the aperture ratio can beimproved.

The pixel configuration shown in FIG. 12 can also suppress variations inthe current value caused by variations in the threshold voltage of thetransistor 110 by operating the pixel in a similar manner to the pixelin FIG. 1. Thus, a current in accordance with luminance data can besupplied to the light emitting element 116, and variations in luminancecan be suppressed. Further, power consumption can be reduced since thepixel is operated with the opposite electrode fixed at a constantpotential. In addition, although the operation region of the transistor110 is not particularly limited, the advantageous effect of theinvention can be more readily obtained when the transistor 110 isoperated in the saturation region. Further, when the transistor 110 isoperated in the saturation region, it is also possible to suppressvariations in the current value flowing to the transistor 110 caused bydeterioration of the light emitting element 116.

Note that a signal from the first scan line 1218 which turns off thefirst switch 111 has a potential of V1−Vth−α (α is an arbitrary positivenumber). Therefore, the first switch 111 has to be a switch which isturned off by a potential of V1−Vth−α (α is an arbitrary positivenumber). In addition, the row of the pixel 1200 has to be operated sothat its initialization period does not overlap with the data writeperiod of the pixel row which shares a wiring with the pixel 1200.

Note that when an n-channel transistor is used as the third switch 113,a potential from the third scan line 120 for turning off the thirdswitch 113 may be set lower than V1−Vth−α which is a signal from thefirst scan line 1218 for turning off the first switch 111. In that case,the gate-source voltage of the transistor in an off state can have anegative value. Thus, current leakage when the third switch 113 is offcan be reduced.

Alternatively, as shown in a pixel 1300 in FIG. 13, a second scan line1319 of a next row can also be used as the second potential supply line123 in FIG. 1. The pixel 1300 can also be operated in a similar mannerto the pixel in Embodiment Mode 1. Note that a signal from the secondscan line 1319 which turns off the second switch 112 has a potential ofV1−Vth−α (α is an arbitrary positive number). Therefore, the secondswitch 112 has to be a switch which is turned off by a potential ofV1−Vth−α (α is an arbitrary positive number). In addition, the row ofthe pixel 1300 has to be operated so that its initialization period doesnot overlap with the data write period of the pixel row which shares awiring with the pixel 1300.

Note that when an n-channel transistor is used as the third switch 113,a potential from the third scan line 120 for turning off the thirdswitch 113 may be set lower than V1−Vth−α which is a signal from thesecond scan line 1319 for turning off the second switch 112. In thatcase, current leakage when the third switch 113 is off can be reduced.

Further, as shown in a pixel 1400 in FIG. 14, a third scan line 1420 ofa preceding row can also be used as the second potential supply line 123in FIG. 1. The pixel 1400 can also be operated in a similar manner tothe pixel in Embodiment Mode 1. Note that a signal from the third scanline 1420 which turns off the third switch 113 has a potential ofV1−Vth−α (α is an arbitrary positive number). Therefore, the thirdswitch 113 has to be a switch which is turned off by a potential ofV1−Vth−α (α is an arbitrary positive number). In addition, the row ofthe pixel 1400 has to be operated so that its initialization period doesnot overlap with the initialization period of the pixel row which sharesa wiring with the pixel 1400; however, when the initialization period isset shorter than the data write period, it is not a big concern.

In addition, when the pixels in FIGS. 12 to 14 are operated in a similarmanner to Embodiment Mode 2, the fourth switch is not particularlyrequired.

Further, as shown in a pixel 1500 in FIG. 15, a fourth scan line 1521 ofa next row can also be used as the second potential supply line 123 inFIG. 1. The pixel 1500 can also be operated in a similar manner to thepixel in Embodiment Mode 1. Note that it is preferable to use the fourthswitch 114 which is turned on when a potential of V1−Vth−α (α is anarbitrary positive number) is input to the fourth scan line 1521. Inthis case, the row of the pixel 1500 has to be operated so that itsinitialization period does not overlap with the data write period of thepixel row which shares a wiring with the pixel 1500. Further, when thefourth switch 114 is turned off in the initialization period, the pixel1500 should be operated so that its initialization period does notoverlap with the initialization period of the pixel row which shares awiring with the pixel 1500.

Note that although this embodiment mode describes the case where thescan line of the next row or the preceding row is also used as thesecond potential supply line 123 in FIG. 1, any other wirings which cansupply a potential of V1−Vth−α (α is an arbitrary positive number) inthe initialization period can be used instead of the second potentialsupply line 123.

Furthermore, the pixel shown in this embodiment mode can be applied tothe display device of FIG. 9. In the display device, start timing of theinitialization period can be freely set in respective rows within therange that the operation of the pixels shown in FIGS. 12 to 15 can beensured and the data write periods in the respective rows do notoverlap. In addition, since each pixel can emit light except in itsaddress period, the ratio of a light emitting period to one frame period(i.e., duty ratio) can be significantly increased and can beapproximately 100%. Therefore, a display device with few luminancevariations and a high duty ratio can be provided.

In addition, since a threshold write period can be set long, thethreshold voltage of the transistor which controls a current valueflowing to the light emitting element can be written into the capacitormore accurately. Therefore, reliability as a display device is improved.

Note that the fourth switch 114 is not necessarily required to beconnected between the node 130 and the gate electrode of the transistor110. It may be connected between the node 130 and the node 131 orbetween the first electrode of the transistor 110 and the node 132.Further, the second electrode of the transistor 110 may be connected tothe power supply line 124 through the fourth switch 114.

This embodiment mode can be freely combined with the pixelconfigurations shown in the other embodiment modes.

Embodiment Mode 5

In this embodiment mode, a pixel having a different configuration fromthat in Embodiment Mode 1 is described with reference to FIG. 29. Notethat portions common to Embodiment Mode 1 and this embodiment mode aredenoted by common reference numerals, and thus detailed description ofthe same portion or a portion having a similar function is omitted.

A pixel shown in FIG. 29 includes a transistor 2910, a first switch 111,a second switch 112, a third switch 113, a fourth switch 114, acapacitor 115, and a light emitting element 116. The pixel is connectedto a signal line 117, a first scan line 118, a second scan line 119, athird scan line 120, a fourth scan line 121, a first potential supplyline 122, a second potential supply line 123, and a power supply line124.

The transistor 2910 in this embodiment mode is a multi-gate transistorwhere two transistors are connected in series, and is provided in thesame position as the transistor 110 in Embodiment Mode 1. Note that thenumber of transistors connected in series is not particularly limited.

By operating the pixel shown in FIG. 29 in a similar manner to the pixelin FIG. 1, variations in the current value caused by variations in thethreshold voltage of the transistor 2910 can be suppressed. Thus, acurrent in accordance with luminance data can be supplied to the lightemitting element 116, and variations in luminance can be suppressed. Inaddition, power consumption can be reduced because operation isperformed with an opposite electrode fixed at a constant potential. Notethat although the operation region of the transistor 2910 is notparticularly limited, the advantageous effect of the invention can bemore readily obtained when the transistor 2910 is operated in thesaturation region.

Further, when the transistor 2910 is operated in the saturation region,it is also possible to suppress variations in the current value flowingto the transistor 2910 caused by deterioration of the light emittingelement 116.

When channel widths of the two transistors connected in series are equalto each other, a channel length L of the transistor 2910 in thisembodiment mode is equal to the sum of the channel lengths of the twotransistors. Thus, a current value which is closer to a constant valuecan be easily obtained in the saturation region regardless of adrain-source voltage Vds. In particular, the transistor 2910 iseffective when it is difficult to manufacture a transistor having a longchannel length L. Note that a connection portion of the two transistorsfunctions as a resistor.

Note that it is acceptable as long as the transistor 2910 has a functionof controlling a current value supplied to the light emitting element116, and the kind of the transistor is not particularly limited.Therefore, a thin film transistor (TFT) using a crystallinesemiconductor film, a thin film transistor using a non-singlecrystalline semiconductor film typified by an amorphous silicon film ora polycrystalline silicon film, a transistor formed using asemiconductor substrate or an SOI substrate, a MOS transistor, ajunction transistor, a bipolar transistor, a transistor using an organicsemiconductor or a carbon nanotube, or other transistors can be used.

In the pixel shown in FIG. 29, transistors can be used as the firstswitch 111, the second switch 112, and the third switch 113, and thefourth switch 114, similarly to the pixel shown in FIG. 1.

Note that the fourth switch 114 is not necessarily required to beconnected between the node 130 and the gate electrode of the transistor110. It may be connected between the node 130 and the node 131 orbetween the first electrode of the transistor 110 and the node 132.Further, the second electrode of the transistor 110 may be connected tothe power supply line 124 through the fourth switch 114.

Further, the fourth switch 114 is not particularly required when thepixel is operated in a manner to Embodiment Mode 2.

Furthermore, the pixel shown in this embodiment mode can be applied tothe display device of FIG. 9. In that case, start timing of theinitialization period can be freely set in respective rows unless datawrite periods in the respective rows overlap, similarly to EmbodimentMode 1. In addition, since each pixel can emit light except in itsaddress period, the ratio of a light emitting period to one frame period(i.e., duty ratio) can be significantly increased and can beapproximately 100%. Therefore, a display device with few luminancevariations and a high duty ratio can be provided.

In addition, since a threshold write period can be set long, thethreshold voltage of the transistor which controls a current valueflowing to the light emitting element can be written into the capacitormore accurately. Therefore, reliability as a display device is improved.

Note that the transistor 2910 is not limited to transistors connected inseries, and a configuration shown in FIG. 30 where transistors areconnected in parallel may be employed. Such a transistor 3010 can supplya larger current to the light emitting element 116. In addition, sincetransistor characteristics are averaged by using two transistorsconnected in parallel, original characteristic variations of thetransistors included in the transistor 3010 can be reduced. With thereduced variations, it becomes easier to suppress variations in thecurrent value caused by variations in the threshold voltage of thetransistor.

This embodiment mode is not limited to the aforementioned description,and can also be applied to any of the pixel configurations shown in theother embodiment modes. That is, the transistor 2910 or the transistor3010 can be applied to the pixel configurations shown in the otherembodiment modes.

Embodiment Mode 6

In this embodiment mode, a pixel configuration which averagesdeterioration of transistors over time by periodically switching thetransistors which control a current value supplied to a light emittingelement in the pixel of the invention is described with reference toFIG. 31.

A pixel shown in FIG. 31 includes a first transistor 3101, a secondtransistor 3102, a first switch 3111, a second switch 3112, a thirdswitch 3113, a fourth switch 3114, a fifth switch 3103, a sixth switch3104, a capacitor 3115, and a light emitting element 3116. The pixel isconnected to a signal line 3117, a first scan line 3118, a second scanline 3119, a third scan line 3120, a fourth scan line 3121, a firstpotential supply line 3122, a second potential supply line 3123, and apower supply line 3124. In addition, although not shown in FIG. 31, thepixel is also connected to fifth and sixth scan lines which controlon/off of the fifth switch 3103 and the sixth switch 3104, respectively.In this embodiment mode, the first transistor 3101 and the secondtransistor 3102 are n-channel transistors, and each transistor is turnedon when a gate-source voltage (Vgs) thereof exceeds the thresholdvoltage. In addition, a pixel electrode of the light emitting element3116 is an anode, and an opposite electrode 3125 thereof is a cathode.Note that a gate-source voltage of the transistor is represented by Vgsand a voltage accumulated in the capacitor is represented by Vcs. Inaddition, the threshold voltage of the first transistor 3101 isrepresented by Vth1 and the threshold voltage of the second transistor3102 is denoted by Vth2. The power supply line 3124, the first potentialsupply line 3122, the second potential supply line 3123, and the signalline 3117 are referred to as a first wiring, a second wiring, a thirdwiring, and a fourth wiring, respectively.

A first electrode (one of a source electrode and a drain electrode) ofthe first transistor 3101 is connected to the pixel electrode of thelight emitting element 3116 through the fifth switch 3103; a secondelectrode (the other of the source electrode and the drain electrode)thereof, to the power supply line 3124; and a gate electrode thereof, tothe first potential supply line 3122 through the fourth switch 3114 andthe second switch 3112. Note that the fourth switch 3114 is connectedbetween the gate electrode of the first transistor 3101 and the secondswitch 3112. In addition, when a connection point of the fourth switch3114 and the second switch 3112 is denoted by a node 3130, the node 3130is connected to the signal line 3117 through the first switch 3111. Inaddition, the first electrode of the first transistor 3101 is alsoconnected to the second potential supply line 3123 through the fifthswitch 3103 and the third switch 3113.

A first electrode (one of a source electrode and a drain electrode) ofthe second transistor 3102 is connected to the pixel electrode of thelight emitting element 3116 through the sixth switch 3104; a secondelectrode (the other of the source electrode and the sprain electrode)thereof, to the power supply line 3124; and a gate electrode thereof, tothe node 3130 through the fourth switch 3114. In addition, the firstelectrode of the second transistor 3102 is also connected to the secondpotential supply line 3123 through the sixth switch 3104 and the thirdswitch 3113. Note that the gate electrode of the first transistor 3101and the gate electrode of the second transistor 3102 are connected toeach other. In addition, the first electrode of the first transistor3101 and the first electrode of the second transistor 3102 are connectedto each other through the fifth switch 3103 and the sixth switch 3104. Aconnection point of the fifth switch 3103 and the sixth switch 3104 isdenoted by a node 3133.

Further, the capacitor 3115 is connected between the node 3133 and thenode 3130. That is, a first electrode of the capacitor 3115 is connectedto the gate electrodes of the first transistor 3101 and the secondtransistor 3102 through the fourth switch 3114, while a second electrodeof the capacitor 3115 is connected to the first electrode of the firsttransistor 3101 through the fifth switch 3103 and to the first electrodeof the second transistor 3102 through the sixth switch 3104. Thecapacitor 3115 may be formed by sandwiching an insulating film between awiring, a semiconductor layer, and an electrode or can be omitted byutilizing the gate capacitance of the first transistor 3101 and thesecond transistor 3102. Note that a connection point of the firstelectrode of the capacitor 3115, the first switch 3111, and the node3130 is denoted by a node 3131, and a connection point of the node 3133,a wiring connected to the second electrode of the capacitor 3115, andthe pixel electrode of the light emitting element 3116 is denoted by anode 3132.

On/off of the first switch 3111, the second switch 3112, the thirdswitch 3113, and the fourth switch 3114 is controlled by inputtingsignals to the first scan line 3118, the second scan line 3119, thethird scan line 3120, and the fourth scan line 3121, respectively. InFIG. 31, scan lines which control on/off of the fifth switch 3103 andthe sixth switch 3104 are omitted.

A signal in accordance with a gray scale level of the pixel whichcorresponds to a video signal, i.e., a potential in accordance withluminance data is input to the signal line 3117.

Next, the operation of the pixel shown in FIG. 31 is described withreference to a timing chart of FIG. 32. Note that one frame period whichcorresponds to a period for displaying an image for one screen in FIG.32 is divided into an initialization period, a threshold write period, adata write period, and a light emitting period.

A potential V1 is input to the opposite electrode 3125 of the lightemitting element 3116 and the first potential supply line 3122, while apotential V1−Vth−α (α: an arbitrary positive number) is input to thesecond potential supply line 3123. Vth corresponds to a higher potentialbetween Vth1 and Vth2. In addition, a potential V2 is input to the powersupply line 3124. Here, the potential of the opposite electrode 3125 ofthe light emitting element 3116 is set equal to the potential of thefirst potential supply line 3122 for descriptive purposes. However,given that the minimum potential difference which is necessary for thelight emitting element 3116 to emit light is represented by V_(EL), itis acceptable as long as the potential of the opposite electrode 3125 ishigher than a potential V1−Vth−α−V_(EL). In addition, it is acceptableas long as the potential V2 of the power supply line 3124 is higher thanthe sum of the potential of the opposite electrode 3125 and the minimumpotential difference (V_(EL)) which is necessary for the light emittingelement 3116 to emit light. However, since the potential of the oppositeelectrode 3125 is set at V1 here for descriptive purposes, it isacceptable as long as V2 is higher than V1+V_(EL).

First, in the initialization period shown in (A) in FIG. 32, the firstswitch 3111 and the sixth switch 3104 are turned off, while the secondswitch 3112, the third switch 3113, the fourth switch 3114, and thefifth switch 3103 are turned on.

At this time, the first electrode of the first transistor 3101 serves asa source electrode, and a potential thereof is V1−Vth−α which is equalto the potential of the second potential supply line 3123. On the otherhand, a potential of the gate electrode of the first transistor 3101 isV1. Thus, a gate-source voltage Vgs of the first transistor 3101 isVth+α and thus the first transistor 3101 is turned on. Then, Vth+α isheld in the capacitor 1215 which is provided between the gate electrodeand the first electrode of the first transistor 3101. Although thefourth switch 3114 shown herein is in an on state, it may be off state.

Next, the third switch 3113 is turned off in the threshold write periodshown in (B) in FIG. 32. Therefore, the potential of the firstelectrode, i.e., the source electrode of the first transistor 3101 risesgradually and when it reaches V1−Vth1, in other words, when thegate-source voltage Vgs of the first transistor 3101 reaches thethreshold voltage (Vth1), the first transistor 3101 is turned off. Thus,the voltage held in the capacitor 3115 becomes Vth1.

In the next data write period shown in (C) in FIG. 32, the second switch3112 and the fourth switch 3114 are turned off, and then the firstswitch 3111 is turned on so that a potential (V1+Vdata) in accordancewith luminance data is input from the signal line 3117. Note that thefirst transistor 3101 can be kept in an off state by turning off thefourth switch 3114. Therefore, potential fluctuations of the secondelectrode of the capacitor 3115, which result from a current suppliedfrom the power supply line 3124 at data writing, can be suppressed. Atthis time, the voltage Vcs held in the capacitor 3115 can be representedby Vth1+Vdata. Note that when the light emitting element 3116 iscontrolled not to emit light in the next light-emitting period, apotential of Vdata≦0 is input.

Next, in the light emitting period shown in (D) in FIG. 32, the firstswitch 3111 is turned off and the fourth switch 3114 is turned on. Atthis time, the gate-source voltage Vgs of the first transistor 3101 isequal to Vth1+Vdata, and thus the first transistor 3101 is turned on.Then, a current in accordance with luminance data flows to the firsttransistor 3101 and the light emitting element 3116, so that the lightemitting element 3116 emits light.

According to such operation, the current flowing to the light emittingelement 3116 can be independent of the threshold voltage (Vth1) of thefirst transistor 3101 regardless of the operation region of the firsttransistor 3101, i.e., the saturation region or the linear region.

Furthermore, in an initialization period of a next frame period shown in(E) in FIG. 32, the fifth switch 3103 is turned off, and the secondswitch 3112, the third switch 3113, the fourth switch 3114, and thesixth switch 3104 are turned on. At this time, the first electrode ofthe second transistor 3102 serves as a source electrode, and a potentialthereof is V1−Vth−α which is equal to the potential of the secondpotential supply line 3123. On the other hand, a potential of the gateelectrode of the second transistor 3102 is V1. Thus, a gate-sourcevoltage Vgs of the second transistor 3102 is Vth+α, and thus the secondtransistor 3102 is turned on. Then, Vth+α is held in the capacitor 3115which is provided between the gate electrode and the first electrode ofthe second transistor 3102. Although the fourth switch 3114 shown hereinis in an on state, it may be off state.

Next, in the threshold write period shown in (F) in FIG. 32, the thirdswitch 3113 is turned off. Therefore, the potential of the firstelectrode, i.e., the source electrode of the second transistor 3102rises gradually and when it reaches V1−Vth2, in other words, when thegate-source voltage Vgs of the second transistor 3102 reaches thethreshold voltage (Vth2), the second transistor 3102 is turned off.Thus, a voltage held in the capacitor 3115 is Vth2.

In the next data write period shown in (G) in FIG. 32, the second switch3112 and the fourth switch 3114 are turned off, and then the firstswitch 3111 is turned on so that a potential (V1+Vdata) in accordancewith luminance data is input from the signal line 3117. Note that thesecond transistor 3102 can be kept in an off state by turning off thefourth switch 3114. Therefore, potential fluctuations of the secondelectrode of the capacitor 3115, which result from a current suppliedfrom the power supply line 3124 at data writing, can be suppressed. Atthis time, the voltage Vcs held in the capacitor 3115 can be representedby Vth2+Vdata.

Next, in the light emitting period shown in (H) in FIG. 32, the firstswitch 3111 is turned off and the fourth switch 3114 is turned on. Atthis time, the gate-source voltage Vgs of the second transistor 3102 isequal to Vth2+Vdata, and thus the second transistor 3102 is turned on.Then, a current in accordance with luminance data flows to the secondtransistor 3102 and the light emitting element 3116, so that the lightemitting element 3116 emits light.

According to such operation, the current flowing to the light emittingelement 3116 can be independent of the threshold voltage (Vth2)regardless of the operation region of the second transistor 3102, i.e.,the saturation region or the linear region.

Therefore, by controlling a current supplied to the light emittingelement 3116 using either the first transistor 3101 or the secondtransistor 3102, variations in the current value caused by variations inthe threshold voltage of the transistor can be suppressed and a currentvalue in accordance with luminance data can be supplied to the lightemitting element 3116. Note that by reducing a load on each transistorby switching between the first transistor 3101 and the second transistor3102, changes in the threshold voltage of the transistor over time canbe reduced.

Accordingly, variations in luminance caused by variations in thethreshold voltage of the first transistor 3101 and the second transistor3102 can be suppressed. In addition, since the potential of the oppositeelectrode is fixed, power consumption can be reduced.

Further, in the case of operating the first transistor 3101 and thesecond transistor 3102 in the saturation region, it is also possible tosuppress variations in current flowing to each transistor caused bydeterioration of the light emitting element 3116.

In the case of operating the first transistor 3101 and the secondtransistor 3102 in the saturation region, the channel lengths L of thesetransistors are preferably long.

In addition, since a reverse bias voltage is applied to the lightemitting element 3116 in the initialization period, a shorted portion ofthe light emitting element can be insulated and deterioration of thelight emitting element can be suppressed. Thus, the lifetime of thelight emitting element can be extended.

Note that since variations in the current value caused by variations inthe threshold voltage of the transistor can be suppressed, a supplydestination of the current controlled by the transistor is notparticularly limited. Therefore, the light emitting element 3116 shownin FIG. 31 can be an EL element (an organic EL element, an inorganic ELelement, or an EL element containing an organic material and aninorganic material), an electron emitting element, a liquid crystalelement, electronic ink, or the like.

In addition, it is acceptable as long as the first transistor 3101 andthe second transistor 3102 have a function of controlling a currentvalue supplied to the light emitting element 3116, and the kind of thetransistors is not particularly limited. Therefore, a thin filmtransistor (TFT) using a crystalline semiconductor film, a thin filmtransistor using a non-single crystalline semiconductor film typified byan amorphous silicon film or a polycrystalline silicon film, atransistor formed using a semiconductor substrate or an SOI substrate, aMOS transistor, a junction transistor, a bipolar transistor, atransistor using an organic semiconductor or a carbon nanotube, or othertransistors can be used.

The first switch 3111 is selected in the timing of inputting a signal inaccordance with a gray scale level of the pixel to the capacitor. Thesecond switch 3112 is selected in the timing of applying a predeterminedpotential to the gate electrode of the first transistor 3101 or thesecond transistor 3102. The third switch 3113 in selected in the timingof applying a predetermined potential for initializing a potentialwritten in the capacitor 3115. The fourth switch 3114 cuts a connectionbetween the gate electrode of the first transistor 3101 or the secondtransistor 3102 and the capacitor 3115. Therefore, the first switch3111, the second switch 3112, the third switch 3113, and the fourthswitch 3114 are not particularly limited as long as they have the abovefunctions. For example, each of the switches may be a transistor, adiode, or a logic circuit combining them. The fifth switch 3103 and thesixth switch 3104 are not particularly limited either, and each of themmay be a transistor, a diode, or a logic circuit combining them.

In the case of using n-channel transistors for the first switch 3111,the second switch 3112, the third switch 3113, the fourth switch 3114,the fifth switch 3103, and the sixth switch 3104, the pixel can beformed using only n-channel transistors; therefore, the manufacturingprocess can be simplified. In addition, a non-crystalline semiconductorsuch as an amorphous semiconductor or a semi-amorphous semiconductor(also referred to as a microcrystalline semiconductor) can be used for asemiconductor layer of each transistor included in the pixel. Forexample, amorphous silicon (a-Si:H) can be used as the amorphoussemiconductor. By using such a non-crystalline semiconductor, themanufacturing process can further be simplified. Accordingly, areduction in manufacturing cost and an improvement in yield can beachieved.

Note that in the case of using transistors for the first switch 3111,the second switch 3112, the third switch 3113, the fourth switch 3114,the fifth switch 3103, and the sixth transistor 3104, the polarity(conductivity type) of the transistors is not particularly limited.However, it is desirable to use a transistor having a characteristic ofsmaller off-current.

It is also possible to switch the position of the first transistor 3101and the fifth switch 3103 as well as the position of the secondtransistor 3102 and the sixth switch 3104 as shown in FIG. 37. That is,the first electrodes of the first transistor 3101 and the secondtransistor 3102 are connected to the gate electrodes of the firsttransistor 3101 and the second transistor 3102 through the capacitor3115. The second electrode of the first transistor 3101 is connected tothe power supply line 3124 through the fifth switch 3103, and the secondelectrode of the second transistor 3102 is connected to the power supplyline 3124 through the sixth switch 3104.

FIGS. 31 and 37 show the examples where the number of elements arrangedin parallel is two, using a transistor and a switch as one set, that is,using the first transistor 3101 and the fifth switch 3103 as a set, andthe second transistor 3102 and the sixth switch 3104 as a set. However,the number of elements arranged in parallel is not particularly limited.

Note that the fourth switch 3114 is necessarily required to be providedbetween the node 3130 and the gate electrodes of the first transistor3101 and the second transistor 3102. It may be connected between thenode 3130 and the node 3131 or between the node 3133 and the node 3132.

Alternatively, the fourth switch 3114 may be omitted as shown in FIG.38. In the pixel shown in this embodiment mode, a current supplied tothe node 3133 from the power supply line 3124 can be stopped withoutusing the fourth switch 3114 but, instead, by turning off the fifthswitch 3103 and the sixth switch 3104 in the data write period.Therefore, potential fluctuations of the second electrode of thecapacitor 3115 can be suppressed, and thus, a voltage of Vth1+Vdata orVth2+Vdata can be held in the capacitor 3115 without providing thefourth switch 3114. Needless to say, the same can be said for aconfiguration shown in FIG. 31 where the fifth switch 3103 is connectedbetween the first electrode of the first transistor 3101 and the node3133 and the sixth switch 3104 is connected between the first electrodeof the second transistor 3102 and the node 3133.

In addition, by turning off both the fifth switch 3103 and the sixthswitch 3104 in the light emitting period, a non-light emitting state canbe forcibly produced. Such operation makes it possible to freely set thelight emitting period. In addition, by inserting black display,afterimages can be made less easily perceived and moving imagecharacteristics can be increased.

Furthermore, the pixel shown in this embodiment mode can be applied tothe display device of FIG. 9. In that case, start timing of theinitialization period can be freely set in respective rows unless datawrite periods in the respective rows overlap, similarly to EmbodimentMode 1. In addition, since each pixel can emit light except in itsaddress period, the ratio of a light emitting period to one frame period(i.e., duty ratio) can be significantly increased and can beapproximately 100%. Therefore, a display device with few luminancevariations and a high duty ratio can be provided.

In addition, since a threshold write period can be set long, thethreshold voltage of the transistor which controls a current valueflowing to the light emitting element can be written into the capacitormore accurately. Therefore, reliability as a display device is improved.

Note that it is possible to use a wiring of another row as the secondpotential supply line 3123 similarly to Embodiment Mode 4. In addition,each of the first transistor 3101 and the second transistor 3102 may bea multi-gate transistor where two transistors are connected in series ormay have a configuration where transistors are connected in parallel.This embodiment mode can be applied not only to such cases but also tothe pixel configurations shown in Embodiment Modes 1 to 5.

Embodiment Mode 7

In this embodiment mode, an example where a p-channel transistor is usedas a transistor for controlling a current value supplied to a lightemitting element is described with reference to FIG. 39.

The pixel shown in FIG. 39 includes a transistor 3910, a first switch3911, a second switch 3912, a third switch 3913, a fourth switch 3914, acapacitor 3915, and a light emitting element 3916. The pixel isconnected to a signal line 3917, a first scan line 3918, a second scanline 3919, a third scan line 3920, a fourth scan line 3921, a firstpotential supply line 3922, a second potential supply line 3923, and apower supply line 3924. In this embodiment mode, the transistor 3910 isa p-channel transistor which is turned on when the absolute value of thegate-source voltage (|Vgs|) thereof exceeds the absolute value of thethreshold voltage (|Vth|) (when Vgs becomes lower than Vth). Inaddition, a pixel electrode of the light emitting element 3916 is acathode and an opposite electrode 3925 thereof is an anode. Note thatthe absolute value of the gate-source voltage of the transistor isrepresented by |Vgs|, and the absolute value of the threshold voltage ofthe transistor is represented by |Vth|. In addition, the power supplyline 3924, the first potential supply line 3922, the second potentialsupply line 3923, and the signal line 3917 are also referred to as afirst wiring, a second wiring, a third wiring, and a fourth wiring,respectively.

A first electrode (one of a source electrode and a drain electrode) ofthe transistor 3910 is connected to the pixel electrode of the lightemitting element 3916; a second electrode (the other of the sourceelectrode and the drain electrode) thereof, to the power supply line3924; and a gate electrode thereof, to the first potential supply line3922 through the fourth switch 3914 and the second switch 3912. Notethat the fourth switch 3914 is connected between the gate electrode ofthe transistor 3910 and the second switch 3912. When a connection pointof the fourth switch 3914 and the second switch 3912 is denoted by anode 3930, the node 3930 is connected to the signal line 3917 throughthe first switch 3911. In addition, the first electrode of thetransistor 3910 is also connected to the second potential supply line3923 through the third switch 3913.

Further, the capacitor 3915 is connected between the node 3930 and thefirst electrode of the transistor 3910. That is, a first electrode ofthe capacitor 3915 is connected to the gate electrode of the transistor3910 through the fourth switch 3914, while a second electrode of thecapacitor 3915 is connected to the first electrode of the transistor3910. The capacitor 3915 may be formed by sandwiching an insulating filmbetween a wiring, a semiconductor layer, and an electrode or can beomitted by utilizing the gate capacitance of the transistor 3910. Aconnection point between the node 3930, the first switch 3911, and thefirst electrode of the capacitor 3931 is denoted by a node 3931, while aconnection point between the first electrode of the transistor 3910, thesecond electrode of the capacitor 3915, and the pixel electrode of thelight-emitting element 3916 is denoted by a node 3932.

Note that on/off of the first switch 3911, the second switch 3912, thethird switch 3913, and the fourth switch 3914 is controlled by inputtingsignals to the first scan line 3918, the second scan line 3919, thethird scan line 3920, and the fourth scan line 3921, respectively.

A signal in accordance with a gray scale level of the pixel whichcorresponds to a video signal, i.e., a potential in accordance withluminance data is input to the signal line 3917.

Next, operation of the pixel shown in FIG. 39 is described withreference to a timing chart in FIG. 40 and FIGS. 41A to 41D. Note thatone frame period which corresponds to a period for displaying an imagefor one screen is divided into an initialization period, a thresholdwrite period, a data write period, and a light emitting period in FIG.40. The initialization period, the threshold write period, and the datawrite period are collectively referred to as an address period. Thelength of one frame period is not particularly limited, but ispreferably 1/60 second or less so that an image viewer does not perceiveflickers.

A potential V1 is input to the opposite electrode 3925 of the lightemitting element 3916 and the first potential supply line 3922, while apotential V1+|Vth|+α (α: an arbitrary positive number) is input to thesecond potential supply line 3923. In addition, a potential V2 is inputto the power supply line 3924.

Here, the potential of the opposite electrode 3925 of the light emittingelement 3916 is set equal to the potential of the first potential supplyline 3922 for descriptive purposes. However, given that the minimumpotential difference which is necessary for the light emitting element3916 to emit light is represented by V_(EL), it is acceptable as long asthe potential of the opposite electrode 3925 is lower than a potentialV1+|Vth|+α+V_(EL). In addition, it is acceptable as long as thepotential V2 of the power supply line 3924 is lower than a potentialwhich is obtained by subtracting the minimum potential difference(V_(EL)) which is necessary for the light emitting element 3916 to emitlight from the potential of the opposite electrode 3925. However, sincethe potential of the opposite electrode 3925 is set at V1 here fordescriptive purposes, it is acceptable as long as V2 is lower thanV1−V_(EL).

First, the first switch 3911 is turned off while the second switch 3912,the third switch 3913, and the fourth switch 3914 are turned on in theinitialization period as shown in (A) in FIG. 40 and FIG. 41A. At thistime, the first electrode of the transistor 3910 serves as a sourceelectrode, and a potential thereof is V1+|Vth|+α which is equal to thepotential of the second potential supply line 3923. On the other hand, apotential of the gate electrode of the transistor 3910 is V1. Thus, theabsolute value of the gate-source voltage |Vgs| of the transistor 3910is |Vth|+α, and thus the transistor 3910 is turned on. Then, |Vth|+α isheld in the capacitor 3915 which is provided between the gate electrodeand the first electrode of the transistor 3910. Although the fourthswitch 3914 shown herein is in an on state, it may be turned off.

Next, the third switch 3913 is turned off in the threshold write periodshown in (B) in FIG. 40 and FIG. 41B. Therefore, the potential of thefirst electrode, i.e., the source electrode of the transistor 3910 risesgradually and when it reaches V1+|Vth|, the transistor 3910 is turnedoff. Thus, the voltage held in the capacitor 3915 becomes |Vth|.

In the next data write period shown in (C) in FIG. 40 and FIG. 41C, thesecond switch 3912 and the fourth switch 3914 are turned off, and thenthe first switch 3911 is turned on so that a potential (V1−Vdata) inaccordance with luminance data is input from the signal line 3917. Notethat the transistor 3910 can be kept in an off state by turning off thefourth switch 3914. Therefore, potential fluctuations of the secondelectrode of the capacitor 3915, which result from a current suppliedfrom the power supply line 3924 at data writing, can be suppressed. Atthis time, the voltage Vcs held in the capacitor 3915 can be representedby Formula (5) where capacitances of the capacitor 3915 and the lightemitting element 3916 are C1 and C2, respectively.

$\begin{matrix}{{Vcs} = {{{- {{Vth}}} - {{Vdata} \times \frac{C\; 2}{{C\; 1} + {C\; 2}}}}}} & (5)\end{matrix}$

Note that C2>>C1 because the light emitting element 3916 is thinner andhas a larger electrode area than the capacitor 3915. Thus, fromC2/(C1+C2)=1, the voltage Vcs held in the capacitor 3915 is representedby Formula (6). Note that when the light emitting element 3916 iscontrolled not to emit light in the next light-emitting period, apotential of Vdata≦0 is input.

Vcs=|−|Vth|−Vdata|  (6)

Next, in the light emitting period shown in (D) in FIG. 40 and FIG. 41D,the first switch 3911 is turned off and the fourth switch 3914 is turnedon. At this time, the gate-source voltage Vgs of the transistor 3910 isequal to −Vdata−|Vth|, and thus the transistor 3901 is turned on. Then,a current in accordance with luminance data flows to the transistor 3910and the light emitting element 3916, so that the light emitting element3916 emits light.

Note that a current I flowing to the light emitting element isrepresented by Formula (7) when the transistor 3910 is operated in thesaturation region.

$\begin{matrix}\begin{matrix}{I = {\frac{1}{2}\left( \frac{W}{L} \right)\mu \; {{Cox}\left( {{Vgs} - {Vth}} \right)}^{2}}} \\{= {\frac{1}{2}\left( \frac{W}{L} \right)\mu \; {{Cox}\left( {{- {Vdata}} - {{Vth}} - {Vth}} \right)}^{2}}}\end{matrix} & (7)\end{matrix}$

Since the transistor 3910 is a p-channel transistor, Vth<0. Therefore,Formula (7) can be transformed to Formula (8).

$\begin{matrix}{I = {\frac{1}{2}\left( \frac{W}{L} \right)\mu \; {{Cox}\left( {- {Vdata}} \right)}^{2}}} & (8)\end{matrix}$

In addition, a current I flowing to the light emitting element isrepresented by Formula (9) when the transistor 3910 is operated in thelinear region.

$\begin{matrix}\begin{matrix}{I = {\left( \frac{W}{L} \right)\mu \; {{Cox}\left\lbrack {{\left( {{Vgs} - {Vth}} \right){Vds}} - {\frac{1}{2}{Vds}^{2}}} \right\rbrack}}} \\{= {\left( \frac{W}{L} \right)\mu \; {{Cox}\left\lbrack {{\left( {{{- V}\; {data}} - {{Vth}} - {Vth}} \right){Vds}} - {\frac{1}{2}{Vds}^{2}}} \right\rbrack}}}\end{matrix} & (9)\end{matrix}$

Since Vth<0, Formula (9) can be transformed to Formula (10).

$\begin{matrix}{I = {\left( \frac{W}{L} \right)\mu \; {{Cox}\left\lbrack {{\left( {- {Vdata}} \right){Vds}} - {\frac{1}{2}{Vds}^{2}}} \right\rbrack}}} & (10)\end{matrix}$

In the formulas, W is the channel width of the transistor 3910; L, thechannel length; μ, mobility; and Cox, accumulated capacitance.

According to Formulas (8) and (10), the current flowing to the lightemitting element 3916 does not depend on the threshold voltage (Vth) ofthe transistor 3910 regardless of the operation region of the transistor3910, i.e., the saturation region or the linear region. Therefore,variations in the current value caused by variations in the thresholdvoltage of the transistor 3910 can be suppressed and a current value inaccordance with luminance data can be supplied to the light emittingelement 3916.

Accordingly, variations in luminance caused by variations in thethreshold voltage of the transistor 3910 can be suppressed. In addition,power consumption can be reduced because the operation is performed withthe opposite electrode fixed at a constant potential.

Furthermore, when the transistor 3910 is operated in the saturationregion, it is also possible to suppress variations in luminance causedby deterioration of the light emitting element 3916. When the lightemitting element 3916 deteriorates, V_(EL) of the light emitting element3916 increases and the potential of the first electrode, i.e., thesource electrode of the transistor 3910 decreases. At this time, thesource electrode of the transistor 3910 is connected to the secondelectrode of the capacitor 3915; the gate electrode of the transistor3910 is connected to the first electrode of the capacitor 3915; and thegate electrode side is in a floating state. Therefore, in accordancewith a decrease in the source potential, the gate potential of thetransistor 3910 also decreases by the same amount. Thus, Vgs of thetransistor 3910 does not change. Therefore, the current flowing to thetransistor 3910 and the light emitting element 3916 is not affected evenif the light emitting element deteriorates. Note that it can also beseen in Formula (8) that the current I flowing to the light emittingelement does not depend on the source potential or the drain potential.

Therefore, when the transistor 3910 is operated in the saturationregion, it is possible to suppress variations in the current valueflowing to the transistor 3910 caused by variations in the thresholdvoltage of the transistor 3910 and deterioration of the light emittingelement 3916.

Note that when the transistor 3910 is operated in the saturation region,the channel length L is of the transistor 3910 is preferably long inorder to suppress an increase in the current amount due to avalanchebreakdown or a channel length modulation.

In addition, since a reverse bias voltage is applied to the lightemitting element 3916 in the initialization period, a shorted portion ofthe light emitting element can be insulated and deterioration of thelight emitting element can be suppressed. Thus, the lifetime of thelight emitting element can be extended.

Note that since variations in the current value caused by variations inthe threshold voltage of the transistor can be suppressed, a supplydestination of the current controlled by the transistor is notparticularly limited. Therefore, the light emitting element 3916 shownin FIG. 39 can be an EL element (an organic EL element, an inorganic ELelement, or an EL element containing an organic material and aninorganic material), an electron emitting element, a liquid crystalelement, electronic ink, or the like.

In addition, it is acceptable as long as the transistor 3910 has afunction of controlling a current value supplied to the light emittingelement 3916, and the kind of the transistor is not particularlylimited. Therefore, a thin film transistor (TFT) using a crystallinesemiconductor film, a thin film transistor using a non-singlecrystalline semiconductor film typified by an amorphous silicon film ora polycrystalline silicon film, a transistor formed using asemiconductor substrate or an SOI substrate, a MOS transistor, ajunction transistor, a bipolar transistor, a transistor using an organicsemiconductor or a carbon nanotube, or other transistors can be used.

The first switch 3911 is selected in the timing of inputting a signal inaccordance with a gray scale level of the pixel to the capacitor, andcontrols a signal supplied to the gate electrode of the transistor 3910.The second switch 3912 is selected in the timing of applying apredetermined potential to the gate electrode of the transistor 3910,and controls whether or not to supply the predetermined potential to thegate electrode of the transistor 3910. The third switch 3913 is selectedin the timing of applying a predetermined potential for initializing apotential written in the capacitor 3915, and sets a potential of thefirst electrode of the transistor 3910 at a high level. The fourthswitch 3914 controls whether or not to connect the gate electrode of thetransistor 3910 and the capacitor 3915. Therefore, the first switch3911, the second switch 3912, the third switch 3913, and the fourthswitch 3914 are not particularly limited as long as they have the abovefunctions. For example, each of the switches may be a transistor, adiode, or a logic circuit combining them.

Note that in the case of using a transistor, a polarity (conductivitytype) thereof is not particularly limited. However, it is desirable touse a transistor having a characteristic of smaller off-current. Asexamples of a transistor with small off-current, there are a transistorprovided with an LDD region, a transistor having a multi-gate structure,and the like. Alternatively, the switch may be a CMOS circuit which usesboth an n-channel transistor and a p-channel transistor.

For example, when p-channel transistors are used as the first switch3911, the second switch 3912, the third switch 3913, and the fourthswitch 3914, an L-level signal is input to the scan lines which controlon/off of the respective switches in order to turn on the switches,while an H-level signal is input in order to turn off the switches.

Since the pixel can be formed using only p-channel transistors in theabove case, the manufacturing process can be simplified.

Furthermore, the pixel shown in this embodiment mode can be applied tothe display device of FIG. 9. In that case, start timing of theinitialization period can be freely set in respective rows unless datawrite periods in the respective rows overlap, similarly to EmbodimentMode 1. In addition, since each pixel can emit light except in itsaddress period, the ratio of a light emitting period to one frame period(i.e., duty ratio) can be significantly increased and can beapproximately 100%. Therefore, a display device with few luminancevariations and a high duty ratio can be provided.

In addition, since a threshold write period can be set long, thethreshold voltage of the transistor which controls a current valueflowing to the light emitting element can be written into the capacitormore accurately. Therefore, reliability as a display device is improved.

Note that this embodiment mode can be freely combined with the pixelconfigurations shown in the other embodiment modes. For example, thefourth switch 3914 may be connected between the node 3930 and the node3931 or between the first electrode of the transistor 3910 and the node3932. Further, the second electrode of the transistor 3910 may beconnected to the power supply line 3924 through the fourth switch 3914.Alternatively, the fourth switch can be omitted as shown in EmbodimentMode 2. This embodiment mode is not limited to the aforementioneddescription, and can also be applied to any of the pixel configurationsshown in the other embodiment modes.

Embodiment Mode 8

In this embodiment mode, one mode of a fragmentary sectional view of apixel of the invention is described with reference to FIG. 17. Note thata transistor shown in the fragmentary sectional view in this embodimentmode is a transistor which has a function of controlling a current valuesupplied to a light emitting element.

First, a base film 1712 is formed over a substrate 1711 having aninsulating surface. As the substrate 1711 having an insulating surface,an insulating substrate such as a glass substrate, a quartz substrate, aplastic substrate (e.g., polyimide, acrylic, polyethylene terephthalate,polycarbonate, polyarylate, polyethersulfone, or the like) or a ceramicsubstrate; or a metal substrate (e.g., tantalum, tungsten, molybdenum,or the like), a semiconductor substrate, or the like which has aninsulating film formed on its surface can be used. Note that it isnecessary to use a substrate which can withstand at least the heatgenerated during a process.

The base film 1712 is formed using a single layer or a plurality oflayers (two or more layers) of an insulating film such as a siliconoxide film, a silicon nitride film, or a silicon oxynitride(SiO_(X)N_(Y)) film. Note that the base film 1712 may be formed by asputtering method, a CVD method, or the like. Although the base film1712 has a single layer in this embodiment mode, it may have a pluralityof layers (two or more layers).

Next, a transistor 1713 is formed over the base film 1712. Thetransistor 1713 includes at least a semiconductor layer 1714, a gateinsulating film 1715 formed over the semiconductor layer 1714, and agate electrode 1716 formed over the semiconductor layer 1714 with thegate insulating film 1715 interposed therebetween. The semiconductorlayer 1714 has a source region and a drain region.

The semiconductor layer 1714 can be formed using a film having anon-crystalline state (i.e., a non-crystalline semiconductor film)selected from an amorphous semiconductor containing amorphous silicon(a-Si:H), silicon, silicon germanium (SiGe), or the like as its maincomponent; a semi-amorphous semiconductor in which an amorphous stateand a crystalline state are mixed; or a microcrystalline semiconductorin which crystal grains of 0.5 to 20 nm can be observed in an amorphoussemiconductor. Alternatively, the semiconductor layer 1714 can also beformed using a crystalline semiconductor film made of polysilicon(p-Si:H) or the like. Note that the microcrystalline state in whichcrystal grains of 0.5 to 20 nm can be observed is called microcrystal.Note that when using a non-crystalline semiconductor film, thesemiconductor layer 1714 may be formed by a sputtering method, a CVDmethod, or the like, and when using a crystalline semiconductor film,the semiconductor layer 1714 may be formed by, for example, forming anon-crystalline semiconductor film and then crystallizing it. Ifnecessary, a slight amount of impurity elements (such as phosphorus,arsenic, or boron) may be contained in the semiconductor layer 1714 inaddition to the above main component in order to control the thresholdvoltage of the transistor.

Next, a gate insulating film 1715 is formed to cover the semiconductorlayer 1714. The gate insulating film 1715 is formed to have either asingle layer or a plurality of stacked layers by using, for example,silicon oxide, silicon nitride, silicon nitride oxide, or the like. Notethat a CVD method, a sputtering method, or the like can be used as afilm deposition method.

Then, a gate electrode 1716 is formed over the semiconductor layer 1714with the gate insulating film 1715 interposed therebetween. The gateelectrode 1716 may be formed to have either a single layer or aplurality of stacked metal films. Note that the gate electrode can beformed using a metal element selected from tantalum (Ta), tungsten (W),titanium (Ti), molybdenum (Mo), aluminum (Al), copper (Cu), or chromium(Cr), or an alloy or compound material containing such an element as amain component. For example, the gate electrode may be formed of a firstconductive film and a second conductive film, using tantalum nitride(TaN) as a first conductive layer and tungsten (W) as a secondconductive layer.

Next, impurities which impart n-type or p-type conductivity areselectively added into the semiconductor layer 1714 using as a mask thegate electrode 1716 or a resist which is formed into a desired shape. Inthis manner, a channel formation region and impurity regions (includinga source region, a drain region, a GOLD region, and an LDD region) areformed in the semiconductor layer 1714. The transistor 1713 can beformed as either an n-channel transistor or a p-channel transistordepending on the conductivity type of the impurity elements that areadded into the semiconductor layer 1714.

Note that in order to form an LDD region 1720 in a self-aligned mannerin FIG. 17, a silicon compound such as a silicon oxide film, a siliconnitride film, or a silicon oxynitride film is formed to cover the gateelectrode 1716 and then etched back to form a sidewall 1717. After that,a source region 1718, a drain region 1719, and an LDD region 1720 can beformed by adding impurities which impart conductivity into thesemiconductor layer 1714. Therefore, the LDD region 1720 is locatedbelow the sidewall 1717. Note that the sidewall 1717 is not necessarilyrequired to be provided because it is only provided to form the LDDregion 1720 in a self-aligned manner. Note that phosphorus, arsenic,boron, or the like is used as the impurities which impart conductivity.

Next, a first interlayer insulating film 1730 is formed by stacking afirst insulating film 1721 and a second insulating film 1722, so as tocover the gate electrode 1716. As the first insulating film 1721 and thesecond insulating film 1722, an inorganic insulating film such as asilicon oxide film, a silicon nitride film, or a silicon oxynitride(SiO_(X)N_(Y)) film or an organic resin film (a photosensitive ornon-photosensitive organic resin film) with a low dielectric constantcan be used. Alternatively, a film containing siloxane may be used. Notethat siloxane is a material having a skeletal structure with the bond ofsilicon (Si) and oxygen (O). As a substituent of siloxane, an organicgroup (e.g., an alkyl group or aromatic hydrocarbon) is used. A fluorogroup may be contained as a substituent as well.

Note that insulating films made of the same material may be used as thefirst insulating film 1721 and the second insulating film 1722. In thisembodiment mode, the first interlayer insulating film 1730 has a stackedstructure of two layers; however, it may have a single layer or astacked structure of three or more layers.

Note that the first insulating film 1721 and the second insulating film1722 may be formed by a sputtering method, a CVD method, a spin coatingmethod, or the like. In the case of using an organic resin film or afilm containing siloxane, a coating method may be employed.

After that, source and drain electrodes 1723 are formed over the firstinterlayer insulating film 1730. The source and drain electrodes 1723are connected to the source region 1718 and the drain region 1719through contact holes, respectively.

Note that the source and drain electrodes 1723 can be formed using ametal such as silver (Ag), gold (Au), copper (Cu), nickel (Ni), platinum(Pt), palladium (Pd), iridium (Ir), rhodium (Rh), tungsten (W), aluminum(Al), tantalum (Ta), molybdenum (Mo), cadmium (Cd), zinc (Zn), iron(Fe), titanium (Ti), silicon (Si), germanium (Ge), zirconium (Zr), orbarium (Ba), an alloy thereof, metal nitride thereof, or stacked filmsthereof.

Next, a second interlayer insulating film 1731 is formed to cover thesource and drain electrodes 1723. As the second interlayer insulatingfilm 1731, an inorganic insulating film, a resin film, or a stackedlayer of such films can be used. As the inorganic insulating film, asilicon nitride film, a silicon oxide film, a silicon oxynitride film,or a stacked layer of such films can be used. For the resin film,polyimide, polyamide, acrylic, polyimide amide, epoxy, or the like canbe used.

A pixel electrode 1724 is formed over the second interlayer insulatingfilm 1731. Next, an insulator 1725 is formed to cover the edge of thepixel electrode 1724. The insulator 1725 is preferably formed to have acurved surface with curvature at an upper end or a lower end thereof inorder to easily deposit a layer 1726 containing a light emittingsubstance later. For example, in the case of using positivephotosensitive acrylic as a material of the insulator 1725, theinsulator 1725 is preferably formed to have a curved surface with acurvature radius (0.2 to 3 μm) only at an upper end. Either a negativeresist which becomes insoluble in an etchant by light irradiation or apositive resist which becomes soluble in an etchant by light irradiationcan be used as the insulator 1725. Further, not only an organic materialbut also an inorganic material such as silicon oxide or siliconoxynitride can be used as a material of the insulator 1725.

Next, a layer 1726 containing a light emitting substance and an oppositeelectrode 1727 are formed over the pixel electrode 1724 and theinsulator 1725.

Note that a light emitting element 1728 is formed in a region where thelayer 1726 containing a light emitting substance is sandwiched betweenthe pixel electrode 1724 and the opposite electrode 1727.

Next, the light emitting element 1728 is described in detail withreference to FIGS. 18A and 18B. Note that the pixel electrode 1724 andthe opposite electrode 1727 in FIG. 17 correspond to a pixel electrode1801 and an opposite electrode 1802 in FIGS. 18A and 18B, respectively.In FIG. 18A, the pixel electrode is an anode and the opposite electrodeis a cathode.

As shown in FIG. 18A, a light emitting layer 1813 as well as a holeinjection layer 1811, a hole transport layer 1812, an electron transportlayer 1814, an electron injection layer 1815, and the like are providedbetween the pixel electrode 1801 and the opposite electrode 1802. Theselayers are stacked so that holes are injected from the pixel electrode1801 side and electrons are injected from the opposite electrode 1802side upon application of a voltage for setting the potential of thepixel electrode 1801 to be higher than the potential of the oppositeelectrode 1802.

In such a light emitting element, the holes injected from the pixelelectrode 1801 and the electrons injected from the opposite electrode1802 are recombined in the light emitting layer 1813 so that a lightemitting substance is excited. Then, the excited light emittingsubstance emits light when returning to a ground state. The lightemitting substance is not specifically limited as long as it can provideluminescence (electroluminescence).

There is no particular limitation on the substance for forming the lightemitting layer 1813 and, therefore, the light emitting layer 1813 maycontain only a light emitting substance. However, when there is apossibility that concentration quenching may occur, the light emittinglayer 1813 is preferably a layer in which a light emitting substance isdispersed in a substance (host) which has a larger energy gap than theenergy gap of the light emitting substance. This can preventconcentration quenching of the light emitting substance. Note that theenergy gap refers to an energy difference between the lowest unoccupiedmolecular orbital (LUMO) level and the highest occupied molecularorbital (HOMO) level.

In addition, there is no particular limitation on the light emittingsubstance, and a substance which can emit light with a desired emissionwavelength may be used. For example, in order to obtain red lightemission, a substance which exhibits light having a peak of an emissionspectrum at 600 to 680 nm can be used, such as4-dicyanomethylene-2-isopropyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbr.: DCJTI),4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbr.: DCJT),4-dicyanomethylene-2-tert-butyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbr.: DCJTB), periflanthene, or2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]benzene.In order to obtain green light emission, a substance which exhibitslight having a peak of an emission spectrum at 500 to 550 nm can beused, such as N,N′-dimethylquinacridon (abbr.: DMQd), coumarin 6,coumarin 545T, tris(8-quinolinolato)aluminum (abbr.: Alq), orN,N′-diphenylquinacridon (DPQd). In order to obtain blue light emission,a substance which exhibits light having a peak of an emission spectrumat 420 to 500 nm can be used, such as9,10-bis(2-naphthyl)-tert-butylanthracene (abbr.: t-BuDNA),9,9′-bianthryl, 9,10-diphenylanthracene (abbr.: DPA),9,10-bis(2-naphthyl)anthracene (abbr.: DNA),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-gallium (abbr.: BGaq),or bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbr.:BAlq).

There is no particular limitation on the substance which is used fordispersing the light emitting substance and, for example, an anthracenederivative such as 9,10-di(2-naphthyl)-2-tert-butylanthracene (abbr.:t-BuDNA), a carbazole derivative such as 4,4′-bis(N-carbazolyl)biphenyl(abbr.: CBP), a metal complex such asbis[2-(2-hydroxyphenyl)pyridinato]zinc (abbr.: Znpp₂) orbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbr.: ZnBOX), or the likecan be used.

Although the anode material for forming the pixel electrode 1801 is notparticularly limited, a metal, an alloy, an electrically conductivecompound, a mixture thereof, or the like which has a high work function(4.0 eV or higher) is preferably used. Specific examples of such ananode material include oxide of a metal material such as indium tinoxide (abbr.: ITO), ITO containing silicon oxide, or indium zinc oxide(abbr.: IZO) which is formed using a target in which indium oxide ismixed with zinc oxide (ZnO) of 2 to 20 wt %. Further, gold (Au),platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), nitride of ametal material (for example, TiN), or the like can be given.

On the other hand, as a substance for forming the opposite electrode1802, a metal, an alloy, an electrically conductive compound, a mixturethereof, or the like which has a low work function (3.8 eV or lower) canbe used. Specific examples of such a cathode material include an elementbelonging to Group 1 or 2 of the Periodic Table, that is, an alkalimetal such as lithium (Li) or cesium (Cs), an alkaline earth metal suchas magnesium (Mg), calcium (Ca), or strontium (Sr), or an alloycontaining these (MgAg or AlLi). In addition, when a layer having anexcellent electron injection property is provided between the oppositeelectrode 1802 and the light emitting layer 1813 such that the layerhaving an excellent electron injection property is in contact with theopposite electrode 1802, the opposite electrode 1802 can be formed byusing various conductive materials including the materials described asthe material for the pixel electrode 1801 such as Al, Ag, ITO, or ITOcontaining silicon oxide, regardless of the magnitude of the workfunction. Alternatively, a similar effect can be obtained by using amaterial having a particularly excellent electron injecting function forforming the electron injection layer 1815 which is described later.

Note that in order to extract light emission to outside, it ispreferable that one or both of the pixel electrode 1801 and the oppositeelectrode 1802 be a transparent electrode made of ITO or the like or beformed with a thickness of several to several tens of nm so that it/theycan transmit visible light.

The hole transport layer 1812 is provided between the pixel electrode1801 and the light emitting layer 1813 as shown in FIG. 18A. The holetransport layer is a layer having a function of transporting holesinjected from the pixel electrode 1801 to the light emitting layer 1813.By providing the hole transport layer 1812 to separate the pixelelectrode 1801 and the light emitting layer 1813 from each other, lightemission can be prevented from being quenched due to metal.

Note that the hole transport layer 1812 is preferably formed using asubstance having an excellent hole transport property, and inparticular, a substance having a hole mobility of 1×10⁻⁶ cm²/Vs orhigher is preferably used. Note that the substance having an excellenthole transport property refers to a substance having a higher mobilityof holes than that of electrons. As specific examples of a substancewhich can be used for forming the hole transport layer 1812, there are4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbr.: NPB),4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbr.: TPD),4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbr.: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbr.:MTDATA),4,4′-bis{N-[4-(N,N-di-m-tolylamino)phenyl]-N-phenylamino}biphenyl(abbr.: DNTPD), 1,3,5-tris[N,N-di(m-tolyl)amino]benzene (abbr.:m-MTDAB), 4,4′,4″-tris(N-carbazolyl)tripheylamine (abbr.: TCTA),phthalocyanine (abbr.: H₂Pc), copper phthalocyanine (abbr.: CuPc),vanadyl phthalocyanine (abbr.: VOPc), and the like. In addition, thehole transport layer 1812 may be a layer having a multilayer structurewhich is formed by combining two or more layers formed of theaforementioned substances.

Further, the electron transport layer 1814 may be provided between theopposite electrode 1802 and the light emitting layer 1813 as shown inFIG. 18A. Here, the electron transport layer is a layer having afunction of transporting electrons injected from the opposite electrode1802 to the light emitting layer 1813. By providing the electrontransport layer 1814 to separate the opposite electrode 1802 and thelight emitting layer 1813 from each other, light emission can beprevented from being quenched due to metal.

There is no particular limitation on the material of the electrontransport layer 1814, and the electron transport layer 1814 can beformed using a metal complex having a quinoline skeleton or abenzoquinoline skeleton such as tris(8-quinolinolato)aluminum (abbr.:Alq), tris(5-methyl-8-quinolinolato)aluminum (abbr.: Almq₃),bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbr.: BeBq₂), orbis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbr.: BAlq),or the like. Alternatively, the electron transport layer 1814 may beformed using a metal complex having an oxazole ligand or a thiazoleligand such as bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbr.:Zn(BOX)₂) Or bis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (abbr.:Zn(BTZ)₂), or the like. Further, the electron transport layer 1814 maybe formed using 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole(abbr.: PBD),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbr.:OXD-7), 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbr.: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbr.: p-EtTAZ), bathophenanthroline (abbr.: BPhen), bathocuproin(abbr.: BCP), or the like. The electron transport layer 1814 ispreferably formed using a substance having a higher mobility ofelectrons than that of holes as described above. In addition, theelectron transport layer 1814 is preferably formed using a substancehaving an electron mobility of 1×10⁻⁶ cm²/Vs or higher. Note that theelectron transport layer 1814 may be a layer having a multilayerstructure which is formed by combining two or more layers formed of theaforementioned substances.

Moreover, the hole injection layer 1811 may be provided between thepixel electrode 1801 and the hole transport layer 1812 as shown in FIG.18A. Here, the hole injection layer refers to a layer having a functionof promoting hole injection from an electrode functioning as an anode tothe hole transport layer 1812.

There is no particular limitation on the material of the hole injectionlayer 1811, and the hole injection layer 1811 can be formed using metaloxide such as molybdenum oxide (MoO_(X)), vanadium oxide (VO_(X)),ruthenium oxide (RuO_(X)), tungsten oxide (WO_(X)), or manganese oxide(MnO_(X)). Alternatively, the hole injection layer 1811 can be formedusing a phthalocyanine-based compound such as phthalocyanine (abbr.:H₂Pc) or copper phthalocyanine (CuPc), an aromatic amine-based compoundsuch as 4,4-bis{N-[4-(N,N-di-m-tolylamino)phenyl]-N-phenylamino}biphenyl(abbr.: DNTPD), a high molecular compound such as a poly(ethylenedioxythiophene)/poly(styrenesulfonic acid) aqueous solution (PEDOT/PSS),or the like.

In addition, a mixture of the aforementioned metal oxide and a substancehaving an excellent hole transport property may be provided between thepixel electrode 1801 and the hole transport layer 1812. Such a layerdoes not cause a rise in drive voltage even when thickened; therefore,optical design using a microcavity effect or a light interference effectcan be conducted by adjusting the thickness of the layer. Therefore, ahigh-quality light emitting element with excellent color purity and fewchanges in color that are dependent on viewing angles can bemanufactured. In addition, the film thickness of such a layer can becontrolled so as to prevent short circuit between the pixel electrode1801 and the opposite electrode 1802 that would occur due toirregularities generated at the film deposition on the surface of thepixel electrode 1801 or due to minute residues remaining on the surfaceof the electrode.

In addition, the electron injection layer 1815 may be provided betweenthe opposite electrode 1802 and the electron transport layer 1814 asshown in FIG. 18A. Here, the electron injection layer is a layer havinga function of promoting electron injection from an electrode functioningas a cathode to the electron transport layer 1814. When the electrontransport layer is not particularly provided, electron injection to thelight emitting layer may be helped by providing the electron injectionlayer between the electrode functioning as a cathode and the lightemitting layer.

There is no particular limitation on the material of the electroninjection layer 1815, and the electron injection layer 1815 can beformed using a compound of alkali metal or alkaline earth metal, such aslithium fluoride (LiF), cesium fluoride (CsF), or calcium fluoride(CaF₂). Alternatively, the electron injection layer 1815 can be formedusing a mixture of a substance having an excellent electron transportproperty such as Alq or 4,4-bis(5-methylbenzoxazol-2-yl)stilbene (BzOs),and alkali metal or alkaline earth metal such as magnesium or lithium.

Note that each of the hole injection layer 1811, the hole transportlayer 1812, the light emitting layer 1813, the electron transport layer1814, and the electron injection layer 1815 may be formed by using anyof an evaporation method, an ink-jet method, a coating method, or thelike. In addition, the pixel electrode 1801 or the opposite electrode1802 may also be formed by using any of a sputtering method, anevaporation method, or the like.

In addition, the layer structure of the light emitting element is notlimited to the one shown in FIG. 18A and, therefore, the light emittingelement may be formed by sequentially forming an electrode functioningas a cathode and forming upper layers thereover as shown in FIG. 18B.That is, the pixel electrode 1801 may be formed as a cathode, and thenthe electron injection layer 1815, the electron transport layer 1814,the light emitting layer 1813, the hole transport layer 1812, the holeinjection layer 1811, and the opposite electrode 1802 may besequentially stacked over the pixel electrode 1801. Note that theopposite electrode 1802 functions as an anode.

Although the light emitting element having a single light emitting layeris shown here, the light emitting element may include a plurality oflight emitting layers. When a plurality of light emitting layers areformed and light emissions from the respective light emitting layers aremixed, white light can be obtained. For example, in the case of forminga light emitting element including two light emitting layers, it ispreferable to provide a spacing layer, a layer which generates holes,and a layer which generates electrons between a first light emittinglayer and a second light emitting layer. This structure enables thelight emitted from the respective light emitting layers to outside to bevisually mixed and perceived as white light. Thus, white light can beobtained.

Light emission is extracted to outside through one or both of the pixelelectrode 1724 and the opposite electrode 1727 in FIG. 17. Accordingly,one or both of the pixel electrode 1724 and the opposite electrode 1727is/are formed of a light-transmitting substance.

When only the opposite electrode 1727 is formed of a light-transmittingsubstance, light emission is extracted from a side opposite to thesubstrate through the opposite electrode 1727 as shown in FIG. 19A. Whenonly the pixel electrode 1724 is formed of a light-transmittingsubstance, light emission is extracted from the substrate side throughthe pixel electrode 1724 as shown in FIG. 19B. When both of the pixelelectrode 1724 and the opposite electro 1727 are formed of alight-transmitting substance, light emission is extracted from both ofthe substrate side and the opposite side thereof through the pixelelectrode 1724 and the opposite electrode 1727 as shown in FIG. 19C.

Next, a transistor having a staggered structure which is formed by usinga non-crystalline semiconductor film for the semiconductor layer of thetransistor 1713 is described. FIGS. 20A and 20B show fragmentarysectional views of pixels. Note that in each of FIGS. 20A and 20B, atransistor having a staggered structure is shown and a capacitorincluded in the pixel is also described in conjunction.

As shown in FIG. 20A, a base film 2012 is formed over a substrate 2011.Further, a pixel electrode 2013 is formed over the base film 2012. Inaddition, a first electrode 2014 is formed of the same material and inthe same layer as the pixel electrode 2013.

Further, a wiring 2015 and a wiring 2016 are formed over the base film2012, and the edge of the pixel electrode 2013 is covered with thewiring 2015. An n-type semiconductor layer 2017 and an n-typesemiconductor layer 2018 each having n-type conductivity are formed overthe wiring 2015 and the wiring 2016, respectively. In addition, asemiconductor layer 2019 is formed over the base film 2012 and betweenthe wiring 2015 and the wiring 2016. A part of the semiconductor layer2019 is extended so as to overlap with the n-type semiconductor layer2017 and the n-type semiconductor layer 2018. Note that thissemiconductor layer is formed of a non-crystalline semiconductor filmmade of an amorphous semiconductor such as amorphous silicon (a-Si:H), asemi-amorphous semiconductor, or a microcrystalline semiconductor. Inaddition, a gate insulating film 2020 is formed over the semiconductorlayer 2019. An insulating film 2021 made of the same material and in thesame layer as the gate insulating film 2020 is also formed over thefirst electrode 2014.

Furthermore, a gate electrode 2022 is formed over the gate insulatingfilm 2020, and thus, a transistor 2025 is formed. In addition, a secondelectrode 2023 made of the same material and in the same layer as thegate electrode 2022 is formed over the first electrode 2014 with theinsulating film 2021 interposed therebetween, and a capacitor 2024 isformed to have a structure where the insulating film 2021 is sandwichedbetween the first electrode 2014 and the second electrode 2023. Aninterlayer insulating film 2026 is formed to cover the edge of the pixelelectrode 2013, the transistor 2025, and the capacitor 2024.

A layer 2027 containing a light emitting substance and an oppositeelectrode 2028 are formed over the interlayer insulating film 2026 andthe pixel electrode 2013 located in an opening of the interlayerinsulating film 2026, and a light emitting element 2029 is formed in aregion where the layer 2027 containing a light emitting substance issandwiched between the pixel electrode 2013 and the opposite electrode2028.

The first electrode 2014 shown in FIG. 20A may be formed of the samematerial and in the same layer as the wirings 2015 and 2016 as shown inFIG. 20B, and a capacitor 2031 may be formed to have a structure wherethe insulating film 2021 is sandwiched between the first electrode 2030and the second electrode 2023. Although an n-channel transistor is usedas the transistor 2025 in FIGS. 20A and 20B, a p-channel transistor mayalso be used.

Materials of the substrate 2011, the base film 2012, the pixel electrode2013, the gate insulating film 2020, the gate electrode 2022, theinterlayer insulating film 2026, the layer 2027 containing a lightemitting substance, and the opposite electrode 2028 may be similar tothose of the substrate 1711, the base film 1712, the pixel electrode1724, the gate insulating film 1715, the gate electrode 1716, theinterlayer insulating films 1730 and 1731, the layer 1726 containing alight emitting substance, and the opposite electrode 1727 shown in FIG.17. In addition, the wiring 2015 and the wiring 2016 may be formed byusing similar materials to those of the source and drain electrodes 1723in FIG. 17.

As another exemplary structure of a transistor which has anon-crystalline semiconductor film as a semiconductor layer, FIGS. 21Aand 21B show fragmentary sectional views of a pixel which includes atransistor with a structure where a gate electrode is sandwiched betweena substrate and a semiconductor layer, i.e., a bottom-gate transistor inwhich a gate electrode is located below a semiconductor layer.

A base film 2112 is formed over a substrate 2111. A gate electrode 2113is formed over the base film 2112. In addition, a first electrode 2114is formed of the same material and in the same layer as the gateelectrode 2113. The gate electrode 2113 may be formed using the samematerial as the gate electrode 1716 shown in FIG. 17, polycrystallinesilicon doped with phosphorus, or silicide that is a compound of metaland silicon.

A gate insulating film 2115 is formed to cover the gate electrode 2113and the first electrode 2114.

A semiconductor layer 2116 is formed over the gate insulating film 2115.A semiconductor layer 2117 made of the same material and in the samelayer as the semiconductor layer 2116 is formed over the first electrode2114. Note that this semiconductor layer is formed of a non-crystallinesemiconductor film made of an amorphous semiconductor such as amorphoussilicon (a-Si:H), a semi-amorphous semiconductor, or a microcrystallinesemiconductor.

An n-type semiconductor layer 2118 and an n-type semiconductor layer2119 each having n-type conductivity are formed over the semiconductorlayer 2116, and an n-type semiconductor layer 2120 is formed over thesemiconductor layer 2117.

A wiring 2121 and a wiring 2122 are formed over the n-type semiconductorlayer 2118 and the n-type semiconductor layer 2119, respectively, andthus a transistor 2129 is formed. A conductive layer 2123 made of thesame material and in the same layer as the wiring 2121 and the wiring2122 is formed over the n-type semiconductor layer 2120. This conductivelayer 2123, the n-type semiconductor layer 2120, and the semiconductorlayer 2117 form a second electrode. Note that a capacitor 2130 is formedwith a structure where the gate insulating film 2115 is sandwichedbetween this second electrode and the first electrode 2114.

One end of the wiring 2121 is extended, and a pixel electrode 2124 isformed on the extended portion of the wiring 2121.

An insulator 2125 is formed to cover an end of the pixel electrode 2124,the transistor 2129, and the capacitor 2130.

A layer 2126 containing a light emitting substance and an oppositeelectrode 2127 are formed over the pixel electrode 2124 and theinsulator 2125, and a light emitting element 2128 is formed in a regionwhere the layer 2126 containing a light emitting substance is sandwichedbetween the pixel electrode 2124 and the opposite electrode 2127.

The semiconductor layer 2117 and the n-type semiconductor layer 2120which serve as a part of the second electrode of the capacitor 2130 arenot particularly required to be provided. That is, a capacitor may beformed to have a structure where the conductive layer 2123 is used asthe second electrode and the gate insulating film 2115 is sandwichedbetween the first electrode 2114 and the conductive layer 2123.

Although an n-channel transistor is used as the transistor 2129, ap-channel transistor may also be used.

Note that when the pixel electrode 2124 is formed before forming thewiring 2121 in FIG. 21A, a capacitor 2132 shown in FIG. 21B can beformed, which has a structure where the gate insulating film 2115 issandwiched between the first electrode 2114 and a second electrode 2131made of the same material and in the same layer as the pixel electrode2124.

Although an inversely staggered transistor with a channel etch structureis described, a transistor with a channel protective structure may beused as well. Next, an example of transistor with a channel protectivestructure is described with reference to FIGS. 22A and 22B. Note thatportions common to FIGS. 21A and 21B and FIGS. 22A and 22B are denotedby common reference numerals.

A transistor 2201 with a channel protective structure shown in FIG. 22Adiffers from the transistor 2129 with a channel etch structure shown inFIG. 21A in that an insulator 2202 serving as an etching mask isprovided over a region of the semiconductor layer 2116 in which achannel is formed.

Similarly, a transistor 2201 with a channel protective structure shownin FIG. 22B differs from the transistor 2129 with a channel etchstructure shown in FIG. 21B in that an insulator 2202 serving as anetching mask is provided over a region of the semiconductor layer 2116in which a channel is formed.

Manufacturing cost can be reduced by using a non-crystallinesemiconductor film for the semiconductor layer of the transistorincluded in the pixel of the invention. Note that the materialsdescribed with reference to FIG. 17 can be used as respective materials.

Structures of the transistor and the capacitor are not limited to thosedescribed above, and transistors and capacitors with various structurescan be used.

In addition, a crystalline semiconductor film made of polysilicon(p-Si:H) or the like as well as a non-crystalline semiconductor filmmade of an amorphous semiconductor such as amorphous silicon (a-Si:H), asemi-amorphous semiconductor, or a microcrystalline semiconductor may beused for the semiconductor layer of the transistor.

Referring now to FIG. 23, a fragmentary sectional view of a pixel whichincludes a transistor having a crystalline semiconductor film as asemiconductor layer is described. Note that a transistor 2318 shown inFIG. 23 is the multi-gate transistor shown in FIG. 29.

As shown in FIG. 23, a base film 2302 is formed over a substrate 2301,and a semiconductor layer 2303 is formed thereover. Note that thesemiconductor layer 2303 is formed by patterning a crystallinesemiconductor film into a desired shape.

An exemplary method of forming the crystalline semiconductor film isdescribed below. First, an amorphous silicon film is deposited over thesubstrate 2301 by a sputtering method, a CVD method, or the like. Thedeposited material is not limited to an amorphous silicon film, and anon-crystalline semiconductor film made of an amorphous semiconductor, asemi-amorphous semiconductor, or a microcrystalline semiconductor may beused. In addition, a compound semiconductor film having an amorphousstructure such as an amorphous silicon germanium film may also be used.

Then, the deposited amorphous silicon film is crystallized using athermal crystallization method, a laser crystallization method, athermal crystallization method using a catalytic element such as nickel,or the like. Thus, a crystalline semiconductor film is obtained. It isalso possible to conduct crystallization by a combining suchcrystallization methods.

In the case of forming a crystalline semiconductor film by a thermalcrystallization method, a heating furnace, laser irradiation, RTA (RapidThermal Annealing), or a method combining them can be used.

When the crystalline semiconductor film is formed by a lasercrystallization method, a continuous wave laser beam (CW laser beam) ora pulsed laser beam can be used. As a laser beam that can be used here,a laser beam emitted from one or more kinds of a gas laser such as an Arlaser, a Kr laser, or an excimer laser; a laser using, as a medium,single-crystal YAG, YVO₄, forsterite (Mg₂SiO₄), YAlO₃, or GdVO₄, orpolycrystalline (ceramic) YAG, Y₂O₃, YVO₄, YAlO₃, or GdVO₄ doped withone or more of Nd, Yb, Cr, Ti, Ho, Er, Tm, and Ta as a dopant; a glasslaser; a ruby laser; an alexandrite laser; a Ti:sapphire laser; a coppervapor laser; and a gold vapor laser can be used. A crystal having alarge grain diameter can be obtained by irradiating the semiconductorfilm with the fundamental wave of the above laser beam or a secondharmonic to a fourth harmonic of the laser beam. For example, the secondharmonic (532 nm) or the third harmonic (355 nm) of a Nd:YVO₄ laser (thefundamental wave: 1064 nm) can be used. At this time, the energy densityof the laser is required to be about 0.01 to 100 MW/cm² (preferably, 0.1to 10 MW/cm²). The scanning rate is set to about 10 to 2000 cm/sec forirradiation.

Note that a laser using, as a medium, single-crystal YAG, YVO₄,forsterite (Mg₂SiO₄), YAlO₃, or GdVO₄, or polycrystalline (ceramic) YAG,Y₂O₃, YVO₄, YAlO₃, or GdVO₄ which is doped with one or more of Nd, Yb,Cr, Ti, Ho, Er, Tm, and Ta as a dopant; an Ar ion laser; or a Ti:sapphire laser can be a CW laser. Alternatively, such a laser can bepulsed at a repetition rate of 10 MHz or higher by conducting Q-switchoperation, mode locking, or the like. When a laser beam is pulsed at arepetition rate of 10 MHz or higher, the semiconductor film isirradiated with the following pulsed laser after being melted by aprevious laser and before being solidified. Therefore, unlike the caseof using a pulsed laser having a low repetition rate, the interfacebetween the solid phase and the liquid phase can be moved continuouslyin the semiconductor film, so that crystal grains that have growncontinuously in the scanning direction can be obtained.

In the case of forming a crystalline semiconductor film by a thermalcrystallization method using a catalytic element such as nickel, it ispreferable to perform gettering treatment after crystallization in orderto remove the catalytic element such as nickel.

By the aforementioned crystallization, a crystallized region is formedin a part of the amorphous semiconductor film. This partly crystallizedcrystalline semiconductor film is patterned into a desired shape,thereby forming an island-shaped semiconductor film. This semiconductorfilm is used for the semiconductor layer 2303 of the transistor.

The crystalline semiconductor layer is used for a channel formationregion 2304 and an impurity region 2305 serving as a source region or adrain region of the transistor 2318 as well as a semiconductor layer2306 and an impurity region 2308 serving as a bottom electrode of acapacitor 2319. Note that the impurity region 2308 is not particularlyrequired to be provided. In addition, channel doping may be performed tothe channel formation region 2304 and the semiconductor layer 2306.

Next, a gate insulating film 2309 is formed over the semiconductor layer2303 and the bottom electrode of the capacitor 2319. Further, a gateelectrode 2310 is formed over the semiconductor layer 2303 with the gateinsulating film 2309 interposed therebetween, and a top electrode 2311made of the same material and in the same layer as the gate electrode2310 is formed over the semiconductor layer 2306 of the capacitor 2319with the gate insulating film 2309 interposed therebetween. In thismanner, the transistor 2318 and the capacitor 2319 are manufactured.

Next, an interlayer insulating film 2312 is formed to cover thetransistor 2318 and the capacitor 2319, and a wiring 2313 is formed overthe interlayer insulating film 2312 so as to be in contact with theimpurity region 2305 through a contact hole. Then, a pixel electrode2314 is formed over the interlayer insulating film 2312 to be in contactwith the wiring 2313, and an insulator 2315 is formed to cover an end ofthe pixel electrode 2314 and the wiring 2313. Further, a layer 2316containing a light emitting substance and an opposite electrode 2317 areformed over the pixel electrode 2314. Thus, a light emitting element2320 is formed in a region where the layer 2316 containing a lightemitting substance is sandwiched between the pixel electrode 2314 andthe opposite electrode 2317.

FIG. 24 shows a fragmentary cross section of a pixel including abottom-gate transistor which uses a crystalline semiconductor film madeof polysilicon (p-Si:H) or the like for a semiconductor layer.

A base film 2402 is formed over a substrate 2401, and a gate electrode2403 is formed thereover. In addition, a first electrode 2404 of acapacitor 2423 is formed of the same material and in the same layer asthe gate electrode 2403.

A gate insulating film 2405 is formed to cover the gate electrode 2403and the first electrode 2404.

A semiconductor layer is formed over the gate insulating film 2405. Notethat the semiconductor layer is formed by crystallizing anon-crystalline semiconductor film made of an amorphous semiconductor, asemi-amorphous semiconductor, a microcrystalline semiconductor, or thelike using a thermal crystallization method, a laser crystallizationmethod, a thermal crystallization method using a catalytic element suchas nickel, or the like, and then patterning the crystallizedsemiconductor film into a desired shape.

Note that the semiconductor layer is used for forming a channelformation region 2406, an LDD region 2407, and an impurity region 2408serving as a source region or a drain region of a transistor 2422, aswell as a region 2409 serving as a second electrode, and impurityregions 2410 and 2411 of the capacitor 2423. Note that the impurityregions 2410 and 2411 are not necessarily required to be provided. Inaddition, the channel formation region 2406 and the region 2409 may bedoped with impurities.

Note that the capacitor 2423 has a structure where the gate insulatingfilm 2405 is sandwiched between the first electrode 2404 and the secondelectrode which includes the region 2409 formed of the semiconductorlayer and the like.

Next, a first interlayer insulating film 2412 is formed to cover thesemiconductor layer, and a wiring 2413 is formed over the firstinterlayer insulating film 2412 so as to be in contact with the impurityregion 2408 through a contact hole.

An opening 2415 is formed in the first interlayer insulating film 2412.A second interlayer insulating film 2416 is formed to cover thetransistor 2422, the capacitor 2423, and the opening 2415, and a pixelelectrode 2417 is formed over the second interlayer insulating film 2416so as to be connected to the wiring 2413 through a contact hole. Inaddition, an insulator 2418 is formed to cover an end of the pixelelectrode 2417. Then, a layer 2419 containing a light emitting substanceand an opposite electrode 2420 are formed over the pixel electrode 2417.Thus, a light emitting element 2421 is formed in a region where thelayer 2419 containing a light emitting substance is sandwiched betweenthe pixel electrode 2417 and the opposite electrode 2420. Note that theopening 2415 is located below the light emitting element 2421. That is,since the first interlayer insulating film 2412 has the opening 2415,transmittance can be increased when light emission from the lightemitting element 2421 is extracted from the substrate side.

By using a crystalline semiconductor film for the semiconductor layer ofthe transistor included in the pixel of the invention, it becomes easierto form, for example, the scan line driver circuit 912 and the signalline driver circuit 911 in FIG. 9 over the same substrate as the pixelportion 913.

Note that the structure of a transistor which uses a crystallinesemiconductor film for a semiconductor layer is not limited to theaforementioned one, and thus the transistor can have various structures.The same can be said for a capacitor. In this embodiment mode, thematerials used in FIG. 17 can be appropriately used unless otherwisementioned.

The transistor shown in this embodiment mode can be used as thetransistor included in the pixel described in any of Embodiment Modes 1to 7, which controls a current value supplied to a light emittingelement. Therefore, variations in the current value caused by variationsin the threshold voltage of the transistor can be suppressed byoperating the pixel similarly to Embodiment Modes 1 to 7. Accordingly, acurrent in accordance with luminance data can be supplied to a lightemitting element, and thus variations in luminance can be suppressed. Inaddition, power consumption can be reduced because operation isperformed with an opposite electrode fixed at a constant potential.

In addition, when such a pixel is applied to the display device of FIG.6, each pixel can emit light except in its address period. Therefore,the ratio of a light emitting period to one frame period (i.e., dutyratio) can be significantly increased and can be approximately 100%.Thus, a display device with few luminance variations and a high dutyratio can be provided.

In addition, since a threshold write period can be set long, thethreshold voltage of the transistor which controls a current valueflowing to the light emitting element can be written into the capacitormore accurately. Therefore, reliability as a display device is improved.

Embodiment Mode 9

In this embodiment mode, one mode of a display device of the inventionis described with reference to FIGS. 25A and 25B.

FIG. 25A is a top view showing a display device, and FIG. 25B is an A-A′line cross sectional view (cross sectional view taken along a line A-A′)of FIG. 25A. The display device includes a signal line driver circuit2501, a pixel portion 2502, a first scan line driver circuit 2503, and asecond scan line driver circuit 2506 over a substrate 2510, which areindicated by dotted lines in the drawing. The display device alsoincludes a sealing substrate 2504 and a sealant 2505, and a portion ofthe display device surrounded by them is a space 2507.

Note that a wiring 2508 is a wiring for transmitting signals to be inputto the first scan line driver circuit 2503, the second scan line drivercircuit 2506, and the signal line driver circuit 2501 and receives videosignals, clock signals, start signals, and the like through an FPC(Flexible Printed Circuit) 2509 which serves as an external inputterminal. IC chips (semiconductor chips provided with a memory circuit,a buffer circuit, and the like) 2518 and 2519 are mounted by COG (ChipOn Glass) or the like on a connection portion of the FPC 2509 and thedisplay device. Although only the FPC is shown here, a printed wiringboard (PWB) may be attached to the FPC. The display device of theinvention includes not only the main body of a display device but also adisplay device with an FPC or a PWB attached thereto. In addition, italso includes a display device mounting an IC chip or the like.

A cross-sectional structure is described with reference to FIG. 25B.Although the pixel portion 2502 and its peripheral driver circuits (thefirst scan line driver circuit 2503, the second scan line driver circuit2506, and the signal line driver circuit 2501) are actually formed overthe substrate 2510, only the signal line driver circuit 2501 and thepixel portion 2502 are shown in the drawing.

Note that the signal line driver circuit 2501 includes transistors withsingle polarity such as n-channel transistors 2520 and 2521. Needless tosay, p-channel transistors may be used or a CMOS circuit may be formedby using not only an n-channel transistor but also a p-channeltransistor. In this embodiment mode, the display panel in which theperipheral driver circuits are formed over the same substrate as thepixel portion is described; however, the invention is not limited tothis. All or part of the peripheral driver circuits may be formed on anIC chip or the like and mounted on the display panel by COG or the like.

The pixel described in any of Embodiment Modes 1 to 7 is used for thepixel portion 2502. Note that FIG. 25B shows a transistor 2511 whichfunctions as a switch, a transistor 2512 which controls a current valuesupplied to a light emitting element, and a light emitting element 2528.Note that a first electrode of the transistor 2512 is connected to apixel electrode 2513 of the light emitting element 2528. In addition, aninsulator 2514 is formed to cover an end of the pixel electrode 2513.Here, the insulator 2514 is formed using a positive photosensitiveacrylic resin film.

The insulator 2514 is formed to have a curved surface with a curvatureat an upper end or a lower end thereof in order to obtain favorablecoverage. For example, in the case of using positive photosensitiveacrylic as a material of the insulator 2514, the insulator 2514 ispreferably formed to have a curved surface with a curvature radius (0.2to 3 μm) only at the upper end. Either a negative resist which becomesinsoluble in an etchant by light irradiation or a positive resist whichbecomes soluble in an etchant by light irradiation can be used as theinsulator 2514.

A layer 2516 containing a light emitting substance and an oppositeelectrode 2517 are formed over the pixel electrode 2513. As long as thelayer 2516 containing a light emitting substance is provided with atleast a light emitting layer, there is no particular limitation onlayers other than the light emitting layer, and thus they can beappropriately selected.

By attaching the sealing substrate 2504 to the substrate 2510 using thesealant 2505, a structure is obtained in which the light emittingelement 2528 is provided in the space 2507 surrounded by the substrate2510, the sealing substrate 2504, and the sealant 2505. Note that thespace 2507 may be filled with either an inert gas (e.g., nitrogen,argon, or the like) or the sealant 2505

Note that an epoxy-based resin is preferably used as the sealant 2505.It is preferable that the material allow as little moisture and oxygenas possible to penetrate therethrough. As a material of the sealingsubstrate 2504, a plastic substrate formed of FRP (Fiberglass-ReinforcedPlastics), PVF (polyvinyl fluoride), Mylar, polyester, acrylic, or thelike can be used other than a glass substrate or a quartz substrate.

By using any of the pixels described in Embodiment Modes 1 to 7 for thepixel portion 2502, variations in luminance among pixels or fluctuationsin luminance of a pixel over time can be suppressed. As a result, adisplay device with a higher duty ratio and higher quality can beobtained. In addition, power consumption can be reduced because theoperation in the invention is performed with the opposite electrodefixed at a constant potential.

By forming all of the signal line driver circuit 2501, the pixel portion2502, the first scan line driver circuit 2503, and the second scan linedriver circuit 2506 over the same substrate as shown in FIGS. 25A and25B, cost of the display device can be reduced. Further, in that case,by using transistors with single polarity for the signal line drivercircuit 2501, the pixel portion 2502, the first scan line driver circuit2503, and the second scan line driver circuit 2506, a manufacturingprocess can be simplified. Accordingly, further cost reduction can beachieved.

The display device of the invention can be obtained in theaforementioned manner. Note that the aforementioned structure is onlyexemplary and the structure of the display device of the invention isnot limited to this.

The display device may also have a structure shown in FIG. 26A where asignal line driver circuit 2601 is formed on an IC chip and the IC chipis mounted on a display device by COG or the like. Note that a substrate2600, a pixel portion 2602, a first scan line driver circuit 2603, asecond scan line driver circuit 2604, an FPC 2605, an IC chip 2606, anIC chip 2607, a sealing substrate 2608, and a sealant 2609 of FIG. 26Acorrespond to the substrate 2510, the pixel portion 2502, the first scanline driver circuit 2503, the second scan line driver circuit 2506, theFPC 2509, the IC chip 2518, the IC chip 2519, the sealing substrate2504, and the sealant 2505 in FIG. 25A, respectively.

That is, only a signal line driver circuit which requires a high speedoperation is formed on an IC chip using a CMOS or the like to reducepower consumption. In addition, higher-speed operation and lower powerconsumption can be achieved by using a semiconductor chip made of asilicon wafer or the like as the IC chip.

Note that by forming the first scan line driver circuit 2603 and thesecond scan line driver circuit 2604 over the same substrate as thepixel portion 2602, cost reduction can be achieved. Moreover, by formingthe first scan line driver circuit 2603, the second scan line drivercircuit 2604, and the pixel portion 2602 using transistors with singlepolarity, further cost reduction can be achieved. At the time, drops inoutput potentials can be prevented by using boot trap circuits for thefirst scan line driver circuit 2603 and the second scan line drivercircuit 2604. In addition, in the case of using amorphous silicon forsemiconductor layers of transistors included in the first scan linedriver circuit 2603 and the second scan line driver circuit 2604, thethreshold voltage of each transistor varies due to deterioration.Therefore, it is preferable to provide a function of correcting thevariations.

By using any of the pixels described in Embodiment Modes 1 to 7 for thepixel portion 2602, variations in luminance among pixels or fluctuationsin luminance of a pixel over time can be suppressed. As a result, adisplay device with a higher duty ratio and higher quality can beobtained. In addition, power consumption can be reduced because theoperation in the invention is performed with the opposite electrodefixed at a constant potential. In addition, a substrate area can beefficiently utilized by mounting an IC chip which is provided with afunctional circuit (a memory or a buffer) on a connection portion of theFPC 2605 and the substrate 2600.

In addition, a structure shown in FIG. 26B may also be employed in whicha signal line driver circuit 2611, a first scan line driver circuit2613, and a second scan line driver circuit 2614 which correspond to thesignal line driver circuit 2501, the first scan line driver circuit2503, and the second scan line driver circuit 2506 of FIG. 25A areformed on IC chips and the IC chips are mounted on a display device byCOG or the like. Note that a substrate 2610, a pixel portion 2612, anFPC 2615, an IC chip 2616, an IC chip 2617, a sealing substrate 2618,and a sealant 2619 of FIG. 26B correspond to the substrate 2510, thepixel portion 2502, the FPC 2509, the IC chip 2518, the IC chip 2519,the sealing substrate 2504, and the sealant 2505 of FIG. 25A,respectively.

In addition, by using a non-crystalline semiconductor film such as anamorphous silicon (a-Si:H) film for the semiconductor layer of thetransistor in the pixel portion 2612, cost reduction can be achieved.Further, it becomes possible to form a large-sized display panel.

Further, the first scan line driver circuit, the second scan line drivercircuit, and the signal line driver circuit are not necessarily requiredto be provided in a row direction and a column direction of the pixels.For example, a peripheral driver circuit 2701 formed on an IC chip asshown in FIG. 27A may incorporate the functions of the first scan linedriver circuit 2613, the second scan line driver circuit 2614, and thesignal line driver circuit 2611 shown in FIG. 26B. Note that a substrate2700, a pixel portion 2702, an FPC 2704, an IC chip 2705, an IC chip2706, a sealing substrate 2707, and a sealant 2708 of FIG. 27Acorrespond to the substrate 2510, the pixel portion 2502, the FPC 2509,the IC chip 2518, the IC chip 2519, the sealing substrate 2504, and thesealant 2505 of FIG. 25A, respectively.

FIG. 27B shows a schematic diagram illustrating the connection ofwirings in the display device of FIG. 27A. FIG. 27B shows a substrate2710, a peripheral driver circuit 2711, a pixel portion 2712, an FPC2713, and an FPC 2714.

The FPC 2713 and the FPC 2714 input signals and power supply potentialsfrom outside to the peripheral driver circuit 2711. Then, an output ofthe peripheral driver circuit 2711 is input to wirings in a rowdirection and a column direction that are connected to pixels includedin the pixel portion 2712.

In the case of using a white light emitting element as a light emittingelement, full-color display can be realized by providing the sealingsubstrate with color filters. The invention can also be applied to sucha display device. FIG. 28 shows an example of a fragmentary sectionalview of a pixel portion.

As shown in FIG. 28, a base film 2802 is formed over a substrate 2800; atransistor 2801 which controls a current value supplied to a lightemitting element is formed thereover; and a pixel electrode 2803 isformed in contact with a first electrode of the transistor 2801. A layer2804 containing a light emitting substance and an opposite electrode2805 are formed thereover.

Note that a portion where the layer 2804 containing a light emittingsubstance is sandwiched between the pixel electrode 2803 and theopposite electrode 2805 serves as the light emitting element. Note thatwhite light is emitted in FIG. 28. A red color filter 2806R, a greencolor filter 2806G, and a blue color filter 2806B are provided above thelight emitting elements in order to achieve full-color display. Inaddition, a black matrix (also referred to as a “BM”) 2807 is providedto separate these color filters.

The display device of this embodiment mode can be appropriately combinedwith the structure shown in Embodiment Mode 8 as well as the structuresshown in Embodiment Modes 1 to 7. In addition, the structure of thedisplay device of the invention is not limited to the aforementionedones, and thus can be applied to a display device having a differentstructure.

Embodiment Mode 10

The display device of the invention can be applied to various electronicdevices. Specifically, it can be applied to display portions ofelectronic devices. Electronic devices include a camera (e.g., a videocamera, a digital camera, or the like), a goggle display, a navigationsystem, an audio-reproducing device (e.g., car audio, an audio componentset, or the like), a computer, a game machine, a portable informationterminal (e.g., a mobile computer, a mobile phone, a portable gamemachine, an electronic book, or the like), an image-reproducing devicehaving a recording medium (specifically, a device for reproducing arecording medium such as a digital versatile disc (DVD) and having adisplay for displaying the reproduced image), and the like.

FIG. 33A shows a display which includes a chassis 3301, a support 3302,a display portion 3303, a speaker portion 3304, a video input terminal3305, and the like.

Note that the pixel shown in any of Embodiment Modes 1 to 7 can be usedfor the display portion 3303. According to the invention, variations inluminance among pixels or fluctuations in luminance of a pixel over timecan be suppressed. As a result, a display having a display portion witha higher duty ratio and higher quality can be obtained. In addition,power consumption can be reduced because the operation in the inventionis performed with an opposite electrode fixed at a constant potential.Note that the display includes in its category all display devices thatare used for displaying information, for example, for a personalcomputer, for TV broadcast reception, for advertisement display, and thelike.

While needs for larger-size displays have been increasing in recentyears, cost increase associated with the increase in display size hasbecome an issue. Therefore, it is an essential task to reduce themanufacturing cost and suppress the price of a high-quality product aslow as possible.

Since the pixel of the invention can be manufactured using transistorswith single polarity, the number of manufacturing steps can be reducedand the manufacturing cost can be reduced. Further, by using anon-crystalline semiconductor film such as an amorphous silicon (a-Si:H)film for a semiconductor layer of each transistor included in the pixel,a process can be simplified and further cost reduction can be achieved.In that case, it is preferable to form a peripheral driver circuit of apixel portion on an IC chip and mount the IC chip on the display panelby COG (Chip On Glass) or the like. Note that it is also possible toform a signal line driver circuit with high speed operation on an ICchip, and form a scan line driver circuit with relatively low speedoperation over the same substrate as the pixel portion, using a circuitincluding transistors with single polarity.

FIG. 33B shows a camera which includes a main body 3311, a displayportion 3312, an image receiving portion 3313, operation keys 3314, anexternal connection port 3315, a shutter 3316, and the like.

Note that the pixel shown in any of Embodiment Modes 1 to 7 can be usedfor the display portion 3312. According to the invention, variations inluminance among pixels or fluctuations in luminance of a pixel over timecan be suppressed. Thus, a camera having a display portion with a higherduty ratio and higher quality can be obtained. In addition, powerconsumption can be reduced because the operation in the invention isperformed with an opposite electrode fixed at a constant potential.

In recent years, competitive manufacturing of digital cameras or thelike has been intensified with an improvement in performance. Therefore,it is vital to suppress the price of high-performance products as low aspossible.

Since the pixel of the invention can be manufactured using transistorswith single polarity, the number of manufacturing steps can be reducedand the manufacturing cost can be reduced. Further, by using anon-crystalline semiconductor film such as an amorphous silicon (a-Si:H)film for a semiconductor layer of each transistor included in the pixel,a process can be simplified and further cost reduction can be achieved.In that case, it is preferable to form a peripheral driver circuit of apixel portion on an IC chip and mount the IC chip on the display panelby COG (Chip On Glass) or the like. Note that it is also possible toform a signal line driver circuit with high speed operation on an ICchip, and form a scan line driver circuit with relatively low speedoperation over the same substrate as the pixel portion, using a circuitincluding transistors with single polarity.

FIG. 33C shows a computer which includes a main body 3321, a chassis3322, a display portion 3323, a keyboard 3324, an external connectionport 3325, a pointing device 3326, and the like. Note that the pixelshown in any of Embodiment Modes 1 to 7 can be used for the displayportion 3323. According to the invention, variations in luminance amongpixels or fluctuations in luminance of a pixel over time can besuppressed. Thus, a computer having a display portion with a higher dutyratio and higher quality can be obtained. In addition, power consumptioncan be reduced because the operation in the invention is performed withan opposite electrode fixed at a constant potential. Further, costreduction can be achieved by using transistors with single polarity asthe transistors included in the pixel portion and using anon-crystalline semiconductor film for semiconductor layers of thetransistors.

FIG. 33D shows a mobile computer which includes a main body 3331, adisplay portion 3332, a switch 3333, operation keys 3334, an infraredport 3335, and the like. Note that the pixel shown in any of EmbodimentModes 1 to 7 can be used for the display portion 3332. According to theinvention, variations in luminance among pixels or fluctuations inluminance of a pixel over time can be suppressed. Thus, a mobilecomputer having a display portion with a higher duty ratio and higherquality can be obtained. In addition, power consumption can be reducedbecause the operation in the invention is performed with an oppositeelectrode fixed at a constant potential. Further, cost reduction can beachieved by using transistors with single polarity as the transistorsincluded in the pixel portion and using a non-crystalline semiconductorfilm for semiconductor layers of the transistors.

FIG. 33E shows a portable image reproducing device provided with arecording medium (specifically, a DVD player), which includes a mainbody 3341, a chassis 3342, a display portion A 3343, a display portion B3344, a recording medium (DVD or the like) reading portion 3345,operation keys 3346, a speaker portion 3347, and the like. The displayportion A 3343 mainly displays image data, while the display portion B3344 mainly displays text data. Note that the pixel shown in any ofEmbodiment Modes 1 to 7 can be used for the display portion A 3343 andthe display portion B 3344. According to the invention, variations inluminance among pixels or fluctuations in luminance of a pixel over timecan be suppressed. Thus, an image reproducing device having a displayportion with a higher duty ratio and higher quality can be obtained. Inaddition, power consumption can be reduced because the operation in theinvention is performed with an opposite electrode fixed at a constantpotential. Further, cost reduction can be achieved by using transistorswith single polarity as the transistors included in the pixel portionand using a non-crystalline semiconductor film for semiconductor layersof the transistors.

FIG. 33F shows a goggle display which includes a main body 3351, adisplay portion 3352, an arm portion 3353, and the like. Note that thepixel shown in any of Embodiment Modes 1 to 7 can be used for thedisplay portion 3352. According to the invention, variations inluminance among pixels or fluctuations in luminance of a pixel over timecan be suppressed. Thus, a goggle display having a display portion witha higher duty ratio and higher quality can be obtained. In addition,power consumption can be reduced because the operation in the inventionis performed with an opposite electrode fixed at a constant potential.Further, cost reduction can be achieved by using transistors with singlepolarity as the transistors included in the pixel portion and using anon-crystalline semiconductor film for semiconductor layers of thetransistors.

FIG. 33G shows a video camera which includes a main body 3361, a displayportion 3362, a chassis 3363, an external connection port 3364, a remotecontrol receiving portion 3365, an image receiving portion 3366, abattery 3367, an audio input portion 3368, operation keys 3369, aneyepiece portion 3360, and the like. Note that the pixel shown in any ofEmbodiment Modes 1 to 7 can be used for the display portion 3362.According to the invention, variations in luminance among pixels orfluctuations in luminance of a pixel over time can be suppressed. Thus,a video camera having a display portion with a higher duty ratio andhigher quality can be obtained. In addition, power consumption can bereduced because the operation in the invention is performed with anopposite electrode fixed at a constant potential. Further, costreduction can be achieved by using transistors with single polarity asthe transistors included in the pixel portion and using anon-crystalline semiconductor film for semiconductor layers of thetransistors.

FIG. 33H shows a mobile phone, which includes a main body 3371, achassis 3372, a display portion 3373, an audio input portion 3374, anaudio output portion 3375, operation keys 3376, an external connectionport 3377, an antenna 3378, and the like. Note that the pixel shown inany of Embodiment Modes 1 to 7 can be used for the display portion 3373.According to the invention, variations in luminance among pixels orfluctuations in luminance of a pixel over time can be suppressed. Thus,a mobile phone having a display portion with a higher duty ratio andhigher quality can be obtained. In addition, power consumption can bereduced because the operation in the invention is performed with anopposite electrode fixed at a constant potential. Further, costreduction can be achieved by using transistors with single polarity asthe transistors included in the pixel portion and using anon-crystalline semiconductor film for semiconductor layers of thetransistors.

As described above, the invention can be applied to various electronicdevices.

Embodiment Mode 11

In this embodiment mode, an exemplary structure of a mobile phone whichincludes the display device of the invention in a display portion isdescribed with reference to FIG. 34.

A display panel 3410 is incorporated in a housing 3400 in a freelydetachable manner. The shape and size of the housing 3400 can be changedas appropriate in accordance with the size of the display panel 3410.The housing 3400 to which the display panel 3410 is fixed is fitted in aprinted circuit board 3401 and assembled as a module.

The display panel 3410 is connected to the printed circuit board 3401through an FPC 3411. The printed circuit board 3401 is provided with aspeaker 3402, a microphone 3403, a transmission/reception circuit 3404,and a signal processing circuit 3405 which includes a CPU, a controller,and the like. Such a module, an input means 3406, and a buttery 3407 arecombined and incorporated in a chassis 3409 and a chassis 3412. Notethat a pixel portion of the display panel 3410 is disposed so that itcan be seen from a window formed in the chassis 3412.

In the display panel 3410, the pixel portion and a part of peripheraldriver circuits (a driver circuit having a low operation frequency amonga plurality of driver circuits) may be formed over the same substrateusing transistors, and another part of the peripheral driver circuits (adriver circuit having a high operation frequency among the plurality ofdriver circuits) may be formed on an IC chip. Then, the IC chip may bemounted on the display panel 3410 by COG (Chip On Glass). Alternatively,the IC chip may be connected to a glass substrate using TAB (TapeAutomated Bonding) or a printed circuit board. Further, all of theperipheral driver circuits may be formed on an IC chip and the IC chipmay be mounted on the display panel by COG or the like.

Note that the pixel shown in any of Embodiment Modes 1 to 7 can be usedfor the pixel portion. According to the invention, variations inluminance among pixels or fluctuations in luminance of a pixel over timecan be suppressed. Thus, the display panel 3410 having a display portionwith a higher duty ratio and higher quality can be obtained. Inaddition, power consumption can be reduced because the operation in theinvention is performed with an opposite electrode fixed at a constantpotential. Further, cost reduction can be achieved by using transistorswith single polarity as the transistors included in the pixel portionand using a non-crystalline semiconductor film for semiconductor layersof the transistors.

The structure of a mobile phone shown in this embodiment mode is onlyexemplary, and the display device of the invention can be applied notonly to the mobile phone having the aforementioned structure but also tomobile phones having various kinds of structures.

Embodiment Mode 12

In this embodiment mode, an EL module obtained by combining a displaypanel and a circuit board is described with reference to FIGS. 35 and36.

Referring to FIG. 35, a display panel 3501 includes a pixel portion3503, a scan line driver circuit 3504, and a signal line driver circuit3505. Over a circuit board 3502, a control circuit 3506, a signaldividing circuit 3507, and the like are formed, for example. Note thatthe display panel 3501 and the circuit board 3502 are connected to eachother by a connection wiring 3508. As the connection wiring 3508, an FPCor the like can be used.

In the display panel 3501, the pixel portion and a part of peripheraldriver circuits (a driver circuit having a low operation frequency amonga plurality of driver circuits) may be formed using transistors over thesame substrate, and another part of the peripheral driver circuits (adriver circuit having a high operation frequency among the plurality ofdriver circuits) may be formed on an IC chip. Then, the IC chip may bemounted on the display panel 3501 by COG (Chip On Glass). Alternatively,the IC chip may be connected to a glass substrate using TAB (TapeAutomated Bonding) or a printed circuit board. Further, all of theperipheral driver circuits may be formed on an IC chip and the IC chipmay be mounted on the display panel by COG or the like.

Note that the pixel shown in any of Embodiment Modes 1 to 7 can be usedfor the pixel portion. According to the invention, variations inluminance among pixels or fluctuations in luminance of a pixel over timecan be suppressed. Thus, the display panel 3501 having a display portionwith a higher duty ratio and higher quality can be obtained. Inaddition, power consumption can be reduced because the operation in theinvention is performed with an opposite electrode fixed at a constantpotential. Further, cost reduction can be achieved by using transistorswith single polarity as the transistors included in the pixel portionand using a non-crystalline semiconductor film for semiconductor layersof the transistors.

An EL TV receiver can be completed with such an EL module. FIG. 36 is ablock diagram showing the main configuration of an EL TV receiver. Atuner 3601 receives video signals and audio signals. A video signal isprocessed by a video signal amplifier circuit 3602, a video signalprocessing circuit 3603 which converts a signal output from the videosignal amplifier circuit 3602 into a color signal corresponding to eachcolor of red, green and blue, and a control circuit 3506 which convertsthe video signal into a signal which meets the input specification of adriver circuit. The control circuit 3506 outputs signals to each of thescan line side and the signal line side. In the case of performing adigital drive, it is possible to adopt a structure in which the signaldividing circuit 3507 is provided on the signal line side so that aninput digital signal is divided into m pieces before being supplied tothe pixel portion.

Among the signals received by the tuner 3601, audio signals aretransmitted to an audio signal amplifier circuit 3604, and an outputthereof is supplied to a speaker 3606 through an audio signal processingcircuit 3605. A control circuit 3607 receives control data on areceiving station (reception frequency) or sound volume from an inputportion 3608 and transmits signals to the tuner 3601 and the audiosignal processing circuit 3605.

By incorporating the EL module in FIG. 35 into the chassis 3301 of FIG.33A which is described in Embodiment Mode 9, a TV receiver can becompleted.

Needless to say, the invention is not limited to the TV receiver, andcan be applied to various uses particularly as a large-sized displaymedium such as an information display board at a train station, anairport, or the like, or an advertisement display board on the street,as well as a monitor of a personal computer.

This application is based on Japanese Patent Application serial no.2005-349780 filed in Japan Patent Office on Dec. 2, 2005, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A semiconductor device comprising: a transistor; afirst switch; a second switch, wherein a first terminal of the secondswitch is electrically connected to a gate of the transistor; a thirdswitch, wherein a first terminal of the third switch is electricallyconnected to one of a source and a drain of the transistor; a fourthswitch, wherein a first terminal of the fourth switch is electricallyconnected to the gate of the transistor; an electrode electricallyconnected to the one of the source and the drain of the transistor; acapacitor, wherein a first terminal of the capacitor is electricallyconnected to the electrode and the one of the source and the drain ofthe transistor and a second terminal of the capacitor is electricallyconnected to a first terminal of the first switch; and a power supplyline electrically connected to the other of the source and the drain ofthe transistor.
 3. The semiconductor device according to claim 2,wherein each of the first switch, the second switch, the fourth switch,and the transistor is a transistor.
 4. The semiconductor deviceaccording to claim 2, wherein each of the first switch, the secondswitch, the fourth switch, and the transistor is an N-channeltransistor.
 5. The semiconductor device according to claim 2, whereinthe transistor has a multi-gate structure in which group at least twotransistors are connected in series.
 6. The semiconductor deviceaccording to claim 2, further comprising a light emitting layer over theelectrode.
 7. The semiconductor device according to claim 6, furthercomprising a sealing substrate over the light emitting layer, whereinthe semiconductor device is configured to emit light from the lightemitting layer through the sealing substrate.
 8. The semiconductordevice according to claim 7, wherein the sealing substrate is providedwith a color filter and the light emitting layer is capable of emittingwhite light.
 9. A display module comprising: a display panel; a circuitboard; and a flexible printed circuit, wherein the display panelcomprises the semiconductor device according to claim
 2. 10. Anelectronic device comprising: a housing; a display panel; a circuitboard; a flexible printed circuit; and a battery, wherein the displaypanel comprises: a transistor; a first switch; a second switch, whereina first terminal of the second switch is electrically connected to agate of the transistor; a third switch, wherein a first terminal of thethird switch is electrically connected to one of a source and a drain ofthe transistor; a fourth switch, wherein a first terminal of the fourthswitch is electrically connected to the gate of the transistor; anelectrode electrically connected to the one of the source and the drainof the transistor; a capacitor, wherein a first terminal of thecapacitor is electrically connected to the electrode and the one of thesource and the drain of the transistor and a second terminal of thecapacitor is electrically connected to a first terminal of the firstswitch; and a power supply line electrically connected to the other ofthe source and the drain of the transistor.
 11. The electronic deviceaccording to claim 10, wherein the electronic device is selected fromthe group consisting of a video camera, a digital camera, a goggledisplay, a navigation system, an audio-reproducing device, a computer, agame machine, a portable information terminal, a mobile computer, amobile phone, a portable game machine, an electronic book, and animage-reproducing device having a recording medium.
 12. A semiconductordevice comprising: a first transistor; a second transistor; a thirdtransistor, wherein a first terminal of the third transistor iselectrically connected to a gate of the first transistor; a fourthtransistor, wherein a first terminal of the fourth transistor iselectrically connected to one of a source and a drain of the firsttransistor; a fifth transistor, wherein a first terminal of the fifthtransistor is electrically connected to the gate of the firsttransistor; a first line electrically connected to a second terminal ofthe second transistor; a second line electrically connected to a secondterminal of the third transistor; a third line electrically connected toa second terminal of the fourth transistor; a fourth line electricallyconnected to the other of the source and the drain of the firsttransistor; an electrode electrically connected to the one of the sourceand the drain of the first transistor; and a capacitor, wherein a firstterminal of the capacitor is electrically connected to the electrode andthe one of the source and the drain of the first transistor, and asecond terminal of the capacitor is electrically connected to a secondterminal of the fifth transistor.
 13. The semiconductor device accordingto claim 12, wherein the first line is a signal line, wherein the secondline is a first potential supply line, wherein the third line is asecond potential supply line, and wherein the fourth line is a powersupply line.
 14. The semiconductor device according to claim 12, furthercomprising a light emitting layer over the electrode.
 15. Thesemiconductor device according to claim 14, further comprising a sealingsubstrate over the light emitting layer, wherein the semiconductordevice is configured to emit light from the light emitting layer throughthe sealing substrate.
 16. The semiconductor device according to claim15, wherein the sealing substrate is provided with a color filter andthe light emitting layer is capable of emitting white light.
 17. Thesemiconductor device according to claim 12, wherein the first transistorhas a multi-gate structure in which at least two transistors areconnected in series.
 18. The semiconductor device according to claim 12,wherein each of the first to fourth transistors is an N-channeltransistor.
 19. A display module comprising: a display panel; a circuitboard; and a flexible printed circuit, wherein the display panelcomprises the semiconductor device according to claim
 12. 20. Anelectronic device comprising: a housing; a display panel; a circuitboard; a flexible printed circuit; and a battery, wherein the displaypanel comprises the semiconductor device according to claim
 12. 21. Theelectronic device according to claim 20, wherein the electronic deviceis selected from the group consisting of a video camera, a digitalcamera, a goggle display, a navigation system, an audio-reproducingdevice, a computer, a game machine, a portable information terminal, amobile computer, a mobile phone, a portable game machine, an electronicbook, and an image-reproducing device having a recording medium.